*(1)* More EM radiation (in the light spectrum and short-wave infra red) is absorbed by the black side of the veins. *(2)* The black side of the vein re-emits longer wavelengths of infra red which are absorbed by air molecules near the surface of the vein, causing thermal expansion. *(3)* The pressure differential causes the veins to be "pushed" with the black face trailing, since this is the side with slightly higher air pressure. *(4)* The air from the white side will obviously be seen creeping to the black side, since this is what happens when you move anything through air. (Some people think "thermal creep" is what _causes_ the veins to move, but it is a symptom of the fact that the veins are being pushed through the air.) _Also, the air pressure needs to be low enough such that the veins can move freely without much resistance, but not so low that there isn't enough air to be heated by the longer wavelengths of infra red being emitted by the black surfaces._
It all makes sense now.. Your explanations really do help a great deal when it comes to understanding this kind of stuff. I think the thing that gets me thinking is how you can stop water dead with a flat surface and 'extract all the momentum' but redirecting it backwards gets even more work out of it. Very interesting, thank you!
Great explanation, especially the part about the best energy transfer when the wheel is rotating at V/2. Same principle for load impedances matched to driver impedances, (loaded voltage = no-load voltage / 2), and maximum power from a motor is when it's loaded to half no-load speed.
This is how I best learn stuff. Lots of hands on experience and experiments supplemented with crucial research. I never was very good at something that requires a lot of reading and research supplemented with crucial experiments. I think that's why I have always struggled with math, yet excel at practical chemistry. Chemistry is heavily math based, but if I am doing something I can actually visualize in front of me, it's easy for me to take measurements, do calculations, and perform successful experiments based on those. I like these videos. Keep them coming (:
Thanks for the great explanation of the Crookes Radiometer! I like that you are practical and show how science applies to everyday life. Something I would like you to explain is why my stainless steel cookware sticks so bad, and what I can do to help alleviate the stick! (Polish or electroplate, or ditch it and buy Teflon!) Keep up the great work!
Should it interest you with some further experimentation, Ben, I propose a method of demonstrating photon pressure on radiometer vanes: No doubt, the needle-and-glass bearing would present too much friction for the extremely weak force. You could create a completely frictionless bearing with a diamagnetically levitated small magnet between two pieces of bismuth (with a much larger magnet held above to counteract gravity, of course). The four vanes would protrude outward from the magnet and bent down to maintain stable center of mass. Mount apparatus in vacuum chamber and pump to high vacuum. Apply bright light and hopefully watch it spin with white sides trailing. ...Or pump to typical radiometer pressure and see how the diamagnetic bearing compares to a needle-and-glass bearing. Would the radiometer be more sensitive with the frictionless bearing?
I really enjoyed this and I'm going to watch it a few more times until I understand it. I really want to understand this. You explained it very well. Thank you.
I’ve seen a lot of explanations of newton’s cradle but never one that explained the two balls kicking up two other balls thing which always fascinated me.
I'm glad someone asked the momentum question from last time, because I left thinking the same thing. Thanks for the awesome explanation and demonstrations! I just saw a kickstarter for a precision top (ForeverSpin), i think the world needs a precision Newton's Cradle, maybe with the cables hung from jewel pivots and amorphous alloy balls.
MrGridStrom Vectors are fun and interesting to work with, especially in 3D. The most fun thing I've done with vectors was calculating the closest point of approach (also referred to as CPA) of 2 moving objects in 3 dimensional space. I also included a constant acceleration vector on each (but not jerk, snap, crackle or pop! I wish I knew enough to calculate those. Those seem extremely complex.) ;-)
Hey Ben, since you've done so much with accelerated electrons, high vacuum and advanced materials processes, may I suggest a future project: an electron beam welder. Certainly a bit of a step up in power level, but think of the experiments!
Hi Ben, just saw your video. About the photon collision/energy transfer, your viewer came up with very interesting question, if you are willing to answer, I suggest looking for Compton effect or Compton Scattering. Again, great video ! Cheers
The key to the Crookes Radiometer is that there is a partial vacuum in the glass sphere. When light hits it, the gas in the sphere, next to the surface of the black side of the vane gets heated up more than the white side. So I assume the gas molecules bounce off the black surface faster than the cooler white surface thus producing thrust. But a good question is, "Since the vane is so thin and presumably made of thermally conductive metal, are the white and black side of a vane the same thermal temperature? If so, then is the difference in emissivity causing the gas to heat more on the black side? Is it radiative heat transfer from the warm black surface to the gas or conductive heat transfer (or possibly convective)? What would happen if the vanes were made of a good thermal insulator? Does it work on the space shuttle or space station?
(In response to the Pelton Wheel): If the water has zero velocity (relative to the pump,) there won't be anywhere for the next "bit" of water to go, right? I know there is something called Betz's Law which refers to the maximum efficiency of this kind of thing, but I'm not 100% sure that this would change the analysis of the optimal speed of the rim of the Pelton Wheel. I'm not trying to be pedantic, I am just curious about this! Thanks for making these videos, you are absolutely amazing at what you do, and I learn a lot from your videos
This just left me with lots of questions that probably come from the fact that I don't know very much about physics. For example, when you talk about holding the radiometer to stop it from moving you say that the photons bounce off the white surface with the same momentum as they came in. But this confuses me because I thought that when you hold it it just acts as if it had more mass and more inertia so it appears to not move but in fact it moves a tiny bit that is almost unmeasurable but it moves. And that that movement would be in proportion to the momentum transfer it received and the mass it has, so it would be like it experiences the same force but it is spread over a much heavier mass so it looks like it didn't move. But if this was case then the photon shouldn't come out with the same momentum as it came in. In fact it should come out with the same momentum as if you weren't holding the radiometer and you allowed it to spin because now that same momentum transfer is being "spread" over a much lighter mass with much less inertia so it spins quickly now but the photons should come out with the same momentum. The problem is that this isn't probably the case since it would not relate to the experiments you showed with the balls I think. So for example if a ball were to collide with another ball at rest of the same mass (and they are both really bouncy so very little energy is transformed into heat and sound), would both balls bounce off at the same speed although that speed would be slower than the original? And if they do and you repeat the experiment (the moving ball moves at the same speed as before) but now the ball at rest is really massive, from what my knowledge tells me the ball should bounce with the same speed as the previous experiment though the heavy ball would move very little. From what the video says the ball should bounce off with almost the same speed as it came in because it is like if if did very little work. But from how I see it the heavy ball has the same kinetic energy as the lighter ball. The problem is that if it is the same then what the video said does not work and I highly doubt that is the case. On the other hand if it is bot the case then things can seem as if energy is not being conserved. Like if I push a car with not much force and the car is not moving with its corresponding kinetic energy then where would that energy be going? Is it really all being converted to heat? Because it doesn't feel right. Also I still don't really see why when he picks up 2 balls from the craddle, the last ball didn't come out with twice the speed but rather always came out the same amount of balls. How is that momentum solves this?
I think the photons warm the black surface. The warm surfaces puts energy into the few air molecules that are present, the energetic air modules exert a force on the black surface.
That's a good point. Usually the speed increase is attributed to the change in moment of inertia, which is true, but it's also true that the skater is doing work because the force to pull arms inward is the same direction as the arms moving inward. Both factors are valid, I think. Another example might be how a child on a swingset can swing higher by moving his/her legs. It more clearly takes work to do this, and causes the swing to go higher.
another source of energy deficiency is the creation of sound energy during collisions. quite hard to measure so don't expect cars bounce like crazy when they crash because too much energy is loss (in example loud sound, the force used up to bend the cars, frictional force)
whats happening in the crookes radiometer is actually pretty simple. The dark absorbent side heats up and gas molecules near it gain heat energy and expand away from the warm vein. This pushes on the vein and causes a torque. Even a radiometer in a good vacuum will probably do this, less gasses for torque but less resistance to motion.
I think you should put the radiometer vane, without its bulb, back into the high vacuum chamber and shine a ridiculously powerful light on it as a final proof that the forces from the photons will push the white side.
Conservation of energy states "energy in" = "energy out". take two frames or reference the "energy in" and the "energy out". "Energy in" would be the potential energy the ball has at the height before you release it, the general equation is "(mass)(gravity)(height)" or "mgh". The second reference frame, "energy out", is the instant right before the ball hits the line of stationary balls. here, all of the potential energy is converted into kinetic energy. Therefore "energy in" = "energy out" or mgh=(1/2)(m)v^2. all the variables but velocity are know so you can solve for it. in theory, assuming there is no energy loss through the ball hitting the line of balls, the ball on the opposite end should leave at the same velocity the original ball hit the line with. or at least I think Im right. you dont necessarily need the momentum equation to solve the problem, you were just missing a piece.
Probably v/2 speed is optimal for wheel because it leaves water with no kinetic energy and incoming jet power is constant (assuming we have little energy loss on water turning inside bucket).
Your videos are awesome. What I don't understand in this one is that since both rubber balls when they hit the sensor come to rest at some point ( before the first bounce that is) this means that they both deccelerated to zero. since the non bouncy obviously took much less time to stop (it didn't have to wait for its deformation) this means that the force vs time must have been much higher and instant, like a Dirac function. So I am guessing something is wrong with the sensor, maybe a threshold or some kind of "inertia" that makes the sensor not responsive enough or a time resolution wasn't dense enough (although I have no idea how the setup works)
Radar guns use the redshift/blueshift of radio waves to measure the speed of cars, so I'm pretty sure you would also get a slight redshift with visible photons hitting a moving surface.
I don't think it's redshift. Redshift is the Doppler effect applied to light, i.e simply a geometrical consequence of the relative speeds of the photon and the observer (the wikipedia article on Doppler is good). Here light is absorbed by the atoms changing their current energy state to a higher one, and then reemitted when they come back to a lower energy state. The energy states of the atoms are quantized (because the electrons jump from one definite "trajectory" to another, there are no "in between" trajectories), so they correspond to specific light frequencies according to the equation E = hf = hc / lambda where f and lambda are respectively the frequency and wavelength and h is Planck's constant. This is what you saw in your home made spectrophotometer: you lit the atoms with light and you got specific wavelengths corresponding to the changes of energy states of the atoms of the material. But they react probabilistically (because everything is probabilistical in the quantum world) only to wavelength corresponding to these changes of energy states. Had you lit the material with a laser with a different wavelength, you would have seen something else or maybe nothing at all (quite unlikely I suppose). So the atoms absorb light selectively. When the photons are absorbed (from the incident direction) and reemitted (in all directions), they transfer their momentum p = h / lambda to the atoms thus creating light pressure. I loved your experiment, BTW.
Interesting behavior of the Newton's cradle. I initially expected it to behave as it did, but your hypothesis made sense too. I was really surprised to see that the behavior was consistent with three and four balls. If you were to attach two of the balls, would it launch only one ball at a greater speed? I think the behavior seen in the experiment was caused by some very small delay between the impact of each ball.
Doesn't compton scattering play a role in it spinning? Is that how you figure out the photons relativistic mass? When I tried that I found the wavelength of the photon not the mass. What if compton scattering causes the green light photons to drop down to IR photons then they hit the black vanes and when they do the heat the black vanes heating the air and causing the black vanes to move along with photons hitting the black vanes directly?
Excellent video. Extremely informative. Personally I'd love to hear you go into relativistic mass... As for the red shift assumption regarding the photons, I'd say that sounds right, but I am also not going to do the math there. ;)
Momentum happens because of mass, among other things. When we search for the source of mass in terms of weight, we turn to gravity. We say that gravity is due to mass, warping space. When we try to find mass, we find energies. So, mass/gravity is some sort of energy, analogous to constant acceleration, which is what a point on a rotating radius experiences, so, is gravity some constant acceleration?
This is very interesting. I have been wondering about the paper representation of what is going on with the spoon shaped part of the wheel. Because the illustration is represented as a 2 dimensional view and we live in 3 dimension in which situations do we need to us a 3 dimensional illustration to comprehend the problem. I assume that water molecules bouncing off in a z axis doesn't affect the physics in this scenario? My perspective is more focused on for example a physics engine point of view to simulate physical behavior to illustrate real life physics.
Another great video Ben! Would you mind turning the audio on your videos into mono in the future? Your camera pics up your voice and moves it from side to side ever so slightly. It's distracting when I'm listening with a headset. Thank you!
so if the Rotor is locked will the light bouncing off be brighter than when it is spinning at half the speed of light because the photons would stop? ( if the plates were curved)
@ninjaonaninja Negative V just refers to the direction. The magnitude of the velocity of V and -V are the same, only the direction is different. Since velocity is a vector quantity it requires a direction. Hope that helps.
Would curving the fins of a radiometer make any discernible difference to its performance? Would it require a directed stream of energy, as with the water jet?
Isn't it all about the time the water has to do its work on the blade?! On the scoop shaped blade the water can push the blade for a longer duration since it is shaped so that it will catch the jet for a longer time, since the angle will change as the wheel turns? A straight blade will very quickly have an angle to the jet (or water fall) that will just make the jet glance off. This is why on a water wheel the flow is directed over the wheel to fall on the far side of the wheel instead of just going under it or on the near side of the water fall. More impeller blades are good!
Yes, I was going to mention this, and ran out of time. Those devices are called anemometers, and can measure wind speed in any direction since the open end of the cup always "catches" more wind than the rounded end.
Forgive me if this has already been addressed. Regarding the pelton wheel, from a practical standpoint, if the wheel was moving at V, thus extracting no energy from the jet of water, the wheel would still be doing its job (crushing wheat or w/e). It would also be doing its job at maximum speed. Of course, after doing its job, the wheel would inevitably slow down, at which point the jet of water would collide with the cups, etc etc.. Am i correct? (possibly splitting hairs on this one, i understand lol)
The only way that the Pelton wheel can be moving at the jet speed (V) is if it is doing no work. At speed V, the wheel will have no torque, so it must be on frictionless bearings and not connected to a wheat grinder or generator. As soon as the wheel does work (grinding wheat, etc), it will slow down. Some electric motors have a similar torque curve, where they produce lots of torque at a standstill, but produce no torque at their free running speed. In fact, their running speed is determined by the point at which they have no torque! If they had more torque to supply, they would continue to turn faster until there was no more torque available.
Applied Science Heh, yeah i realized that a bit after making the comment. But what you mentioned about torque determining the running speed of an electric motor has given me something to think about. I'm working on a V.A.W.T. (vertical axis wind turbine) using an electric motor as the generator and torque is something I didn't have a full grasp on until tonight. Thanks Ben.
So when you changed the pressure around the Radiometer, you changed the light velocity. Would that mean the mass changed as well. Did it not "optimize" at partial pressure? How would variations in atm pressure effect a solar cells eff?
I have found that speed and mass and kinetic energy can be explained much like a container with water flowing out of it from the bottom. Speed/kinetic energy can be described as an amount of water and the container, it's mass. A % of the container being full, Say 50% correlates to 100mph, it takes much less energy/water to fill that mass to 50% of the mass is small, than of its a large mass. Once the container is full, you only need to keep up with the rate at which the energy leaks out. That is why it takes more energy to get up to speed, than to keep a speed. You have to fill the container. Think about this, there are much more ways that this lines up.
Could you run a water wheel from city water and use the generated power to pump that water into a tank? Would there be any real value power generation wise if you did?
I imagine you would actually lose power if you did that. Why wouldn't you just pour the water into the tank straight from the tap? Adding the extra water-wheel/pump contraption seems like and unnecessary additional step.
Haha. I came up with the idea of a Pelton Wheel the other day. I was at an old mill and was looking at the blades and wondered why they didn't curve the ends in the shape of a scoop to make it more efficient. I guess they did.
If photons always travel at the same speed, why would they be red-shifted? Surely that is a function of wave-length, which wouldn't be changed. If you shine, say blue light on the radiometer. will it rotate at the same speed as red light?
Question. Which of these two scenarios has the higher kinetic energy? Two cars colliding head on at 60 mph each or one car hitting an immovable object like a bridge abutment at 60 mph? REPLY
So the red line is like measuring Horse Power but with some other unit but the same way to measure force? My brain is hurting trying to figure out an analogy/equivalent to what the red line is. I have an analog scope (don't know much about them fancy pants new fangled dig scopes) so the red line is calculated by some math function in the scope? If you had units for the red line what would it be newtons/time? or newtons/length?
I sometimes work with high power photo flashes. But if I hang a sheet of metal (1,5 mm aluminium) in front of it and trigger the flash, the sheet will "chime" just by the light. I have always been wondering if it's the photons bouncing of like your video or if it is a thermal phenomena due to the high energy released from the flash tube. Any thoughts?
How would you get the Pelton wheel to rotate at v/2? I understand why it is the best operating speed, but how would you keep the wheel spinning at v/2? I could think of adding a damper to the system, but then you're just bleeding energy, and then is it still as efficient? Or will the wheel naturally spin at v/2 because that's when it gets the most force (change in momentum)? Is it something like if you plot input water speed versus wheel speed (assuming they're along the same direction), is there some sort of stable maximum/minimum?
h0lx Yeah I was thinking about that, but then you'd need a controller (or something) to sense the velocity of both the wheel and the incident flow and be able to actuate the nozzle. I guess you could do it by hand and approximate it, I'm just not sure how exactly it's done.
WELL, THIS MAY BE DUMB BUT.... I think there are air molecules above the vanes. The dark vanes warm the air. The warm molecules fly away from the vanes exerting force which causes the vanes to move away.
While I was looking at your drawing of the water entering the scoops it occurred to me that the scoops would enter and leave the stream without being in the optimal position for very long. What would the best arrangement be to make a water wheel like that in real life? 6 cups and 5 jets to stagger the forces? Perhaps you could make something simple to show us.
I think most commercial Pelton wheels have many more buckets, and often more than one water jet. There are some photos here: en.wikipedia.org/wiki/Pelton_wheel
If I want to break open a door with a baseball bat, should I use a heavier bat ( more momentum) or a standard bat but swing it faster ( velocity squared and more kinetic energy) , what would be more effective?
Hi, a shorter wave length = higher frequency = more energy. This means a red shift is a lengthening because the wave loses energy. Since the speed of light is constant within a medium it has to lose energy that way. A magnifying glass however would itself not act as a nozzle because the light is not only shifted in color but has also changed its speed. The speed of light depends on the refractive index of a medium! This means the total energy for the light stays the same within the glass and it leaves the glass as it came in. If red light comes in, it is blue shifted and slowed down within the glass. Leaving the glass it is sped up and red shifted again.
There not such a literal analogy for the nosle as the light does not need to be placed on a specific part of the radiometer vane. The amount of energy transferred depends on the reflectivity of the radiometer vane which is why this is different on either side so that more force is applied to one side than the other and so it will spin. The analogy is between the cup changing the direction of the water by directing the flow to the opposite direction and the reflectivity of the radiometer vanes changing the direction of the light by reflecting it.
A little over 7 minutes in you blew my fucking mind. I never once considered that sound is energy. I mean - if someone asked me that before I'd say yes, I just wouldn't put 2 and 2 together in the bouncy ball experiment you did.
When you form the kinetic energy from the velocity, you aren't really "squaring the velocity vector" -- you are taking the scalar product of the velocity vector with itself to obtain the square of its speed. Us mechanician's like to be very precise with these things.
My radiometer works from the radiant heat of my gas fire, I assume it's the heat and not the light. It would be interesting to see if it works from the heat of a hot plate that doesn't emit light.
Where does the kinetic energy go? If there is an 80kg target that completely absorbs the .025 kg bullet moving 300 m/s the momentum equation gives the target about 0.1 m/s velocity and .4 joules kinetic energy. What happens to the 1000+ joules of energy the projectile had on impact (assuming target absorbed it and the bullet didnt continue traveling past target)? Please help this make more sense - thank you
All the energy in this case is used to bring the bullet to a stop. If the target did actually move, the bullet would still have some momentum forward (thus it would be moving forward). If the bullet was elastic, then i would store the energy and bounce. Its important to consider whether a collision is elastic or not, before using the momentum equation.
Great video. I just reviewed MIT OCW 8.01 Lec 15,16,17 by Walter Lewin just the other day. This is an awesome application. May try and work through it tomorrow or soon.
You may have been the last person to view that lecture. All the online resources have all been removed indefinitely due to a sexual harassment investigation: newsoffice.mit.edu/2014/lewin-courses-removed-1208
Applied Science Man, i just searched youtube and the opencourseware 8.01 series was there, I hit refresh a couple min later and it was gone. :( Other people have the series up still... probably until MIT makes them take it doen
ok my brain is almost done , one more video like this and should be ready to serve
We "found it enjoyable". Thanks.
*(1)* More EM radiation (in the light spectrum and short-wave infra red) is absorbed by the black side of the veins.
*(2)* The black side of the vein re-emits longer wavelengths of infra red which are absorbed by air molecules near the surface of the vein, causing thermal expansion.
*(3)* The pressure differential causes the veins to be "pushed" with the black face trailing, since this is the side with slightly higher air pressure.
*(4)* The air from the white side will obviously be seen creeping to the black side, since this is what happens when you move anything through air. (Some people think "thermal creep" is what _causes_ the veins to move, but it is a symptom of the fact that the veins are being pushed through the air.)
_Also, the air pressure needs to be low enough such that the veins can move freely without much resistance, but not so low that there isn't enough air to be heated by the longer wavelengths of infra red being emitted by the black surfaces._
It all makes sense now.. Your explanations really do help a great deal when it comes to understanding this kind of stuff. I think the thing that gets me thinking is how you can stop water dead with a flat surface and 'extract all the momentum' but redirecting it backwards gets even more work out of it. Very interesting, thank you!
Very nice video. It's always nice to see the fundamental driving principles behind what sometimes seems obvious, or non obvious in some cases.
Ben, you probably have the best YT-channel for a science/technology loving guy like me.
Keep up the good and awsome work!
Great explanation, especially the part about the best energy transfer when the wheel is rotating at V/2. Same principle for load impedances matched to driver impedances, (loaded voltage = no-load voltage / 2), and maximum power from a motor is when it's loaded to half no-load speed.
This is how I best learn stuff. Lots of hands on experience and experiments supplemented with crucial research. I never was very good at something that requires a lot of reading and research supplemented with crucial experiments.
I think that's why I have always struggled with math, yet excel at practical chemistry. Chemistry is heavily math based, but if I am doing something I can actually visualize in front of me, it's easy for me to take measurements, do calculations, and perform successful experiments based on those.
I like these videos. Keep them coming (:
This guy always manage to blow my mind every time. Seems he always has a fresh view that I havent heard before
Thanks for the great explanation of the Crookes Radiometer! I like that you are practical and show how science applies to everyday life. Something I would like you to explain is why my stainless steel cookware sticks so bad, and what I can do to help alleviate the stick! (Polish or electroplate, or ditch it and buy Teflon!) Keep up the great work!
Should it interest you with some further experimentation, Ben, I propose a method of demonstrating photon pressure on radiometer vanes:
No doubt, the needle-and-glass bearing would present too much friction for the extremely weak force. You could create a completely frictionless bearing with a diamagnetically levitated small magnet between two pieces of bismuth (with a much larger magnet held above to counteract gravity, of course). The four vanes would protrude outward from the magnet and bent down to maintain stable center of mass. Mount apparatus in vacuum chamber and pump to high vacuum. Apply bright light and hopefully watch it spin with white sides trailing.
...Or pump to typical radiometer pressure and see how the diamagnetic bearing compares to a needle-and-glass bearing. Would the radiometer be more sensitive with the frictionless bearing?
How sensitive a detector would we need to measure the red shift? That would be really cool to see too.
fsmv depends on the degree of red shift
Very good and intuitive talk on momentum - thank you!
I really enjoyed this and I'm going to watch it a few more times until I understand it. I really want to understand this. You explained it very well. Thank you.
I’ve seen a lot of explanations of newton’s cradle but never one that explained the two balls kicking up two other balls thing which always fascinated me.
This was definitely a longer form than your usual videos, but I rather liked it.
I'm glad someone asked the momentum question from last time, because I left thinking the same thing. Thanks for the awesome explanation and demonstrations!
I just saw a kickstarter for a precision top (ForeverSpin), i think the world needs a precision Newton's Cradle, maybe with the cables hung from jewel pivots and amorphous alloy balls.
You only need heavier steel balls with fishing line as strings.
Great video. Nice lab/workshop you have.
I never knew velocity was a vector, alot of stuff i know just clicked; this is amazing.
MrGridStrom Vectors are fun and interesting to work with, especially in 3D. The most fun thing I've done with vectors was calculating the closest point of approach (also referred to as CPA) of 2 moving objects in 3 dimensional space. I also included a constant acceleration vector on each (but not jerk, snap, crackle or pop! I wish I knew enough to calculate those. Those seem extremely complex.) ;-)
Hey Ben, since you've done so much with accelerated electrons, high vacuum and advanced materials processes, may I suggest a future project: an electron beam welder. Certainly a bit of a step up in power level, but think of the experiments!
Hi Ben, just saw your video. About the photon collision/energy transfer, your viewer came up with very interesting question, if you are willing to answer, I suggest looking for Compton effect or Compton Scattering.
Again, great video !
Cheers
The key to the Crookes Radiometer is that there is a partial vacuum in the glass sphere. When light hits it, the gas in the sphere, next to the surface of the black side of the vane gets heated up more than the white side. So I assume the gas molecules bounce off the black surface faster than the cooler white surface thus producing thrust. But a good question is, "Since the vane is so thin and presumably made of thermally conductive metal, are the white and black side of a vane the same thermal temperature? If so, then is the difference in emissivity causing the gas to heat more on the black side? Is it radiative heat transfer from the warm black surface to the gas or conductive heat transfer (or possibly convective)? What would happen if the vanes were made of a good thermal insulator? Does it work on the space shuttle or space station?
(In response to the Pelton Wheel): If the water has zero velocity (relative to the pump,) there won't be anywhere for the next "bit" of water to go, right? I know there is something called Betz's Law which refers to the maximum efficiency of this kind of thing, but I'm not 100% sure that this would change the analysis of the optimal speed of the rim of the Pelton Wheel. I'm not trying to be pedantic, I am just curious about this! Thanks for making these videos, you are absolutely amazing at what you do, and I learn a lot from your videos
Great video! Better explanation than the one I got in High School when we were taught linear momentum, :)
This just left me with lots of questions that probably come from the fact that I don't know very much about physics. For example, when you talk about holding the radiometer to stop it from moving you say that the photons bounce off the white surface with the same momentum as they came in. But this confuses me because I thought that when you hold it it just acts as if it had more mass and more inertia so it appears to not move but in fact it moves a tiny bit that is almost unmeasurable but it moves. And that that movement would be in proportion to the momentum transfer it received and the mass it has, so it would be like it experiences the same force but it is spread over a much heavier mass so it looks like it didn't move. But if this was case then the photon shouldn't come out with the same momentum as it came in. In fact it should come out with the same momentum as if you weren't holding the radiometer and you allowed it to spin because now that same momentum transfer is being "spread" over a much lighter mass with much less inertia so it spins quickly now but the photons should come out with the same momentum.
The problem is that this isn't probably the case since it would not relate to the experiments you showed with the balls I think.
So for example if a ball were to collide with another ball at rest of the same mass (and they are both really bouncy so very little energy is transformed into heat and sound), would both balls bounce off at the same speed although that speed would be slower than the original?
And if they do and you repeat the experiment (the moving ball moves at the same speed as before) but now the ball at rest is really massive, from what my knowledge tells me the ball should bounce with the same speed as the previous experiment though the heavy ball would move very little. From what the video says the ball should bounce off with almost the same speed as it came in because it is like if if did very little work. But from how I see it the heavy ball has the same kinetic energy as the lighter ball.
The problem is that if it is the same then what the video said does not work and I highly doubt that is the case.
On the other hand if it is bot the case then things can seem as if energy is not being conserved. Like if I push a car with not much force and the car is not moving with its corresponding kinetic energy then where would that energy be going? Is it really all being converted to heat? Because it doesn't feel right.
Also I still don't really see why when he picks up 2 balls from the craddle, the last ball didn't come out with twice the speed but rather always came out the same amount of balls. How is that momentum solves this?
I think the photons warm the black surface. The warm surfaces puts energy into the few air molecules that are present, the energetic air modules exert a force on the black surface.
We can estimate light pressure directing laser beam on black and white sides of the radiometer and measuring speed of rotation of the blades.
Nice explanation. And when the skater pulls in her arms that requires work/energy, same amount as the kinetic energy increase, right?
That's a good point. Usually the speed increase is attributed to the change in moment of inertia, which is true, but it's also true that the skater is doing work because the force to pull arms inward is the same direction as the arms moving inward. Both factors are valid, I think. Another example might be how a child on a swingset can swing higher by moving his/her legs. It more clearly takes work to do this, and causes the swing to go higher.
another source of energy deficiency is the creation of sound energy during collisions. quite hard to measure so don't expect cars bounce like crazy when they crash because too much energy is loss (in example loud sound, the force used up to bend the cars, frictional force)
whats happening in the crookes radiometer is actually pretty simple. The dark absorbent side heats up and gas molecules near it gain heat energy and expand away from the warm vein. This pushes on the vein and causes a torque. Even a radiometer in a good vacuum will probably do this, less gasses for torque but less resistance to motion.
I think you should put the radiometer vane, without its bulb, back into the high vacuum chamber and shine a ridiculously powerful light on it as a final proof that the forces from the photons will push the white side.
Conservation of energy states "energy in" = "energy out". take two frames or reference the "energy in" and the "energy out". "Energy in" would be the potential energy the ball has at the height before you release it, the general equation is "(mass)(gravity)(height)" or "mgh". The second reference frame, "energy out", is the instant right before the ball hits the line of stationary balls. here, all of the potential energy is converted into kinetic energy. Therefore "energy in" = "energy out" or mgh=(1/2)(m)v^2. all the variables but velocity are know so you can solve for it. in theory, assuming there is no energy loss through the ball hitting the line of balls, the ball on the opposite end should leave at the same velocity the original ball hit the line with. or at least I think Im right. you dont necessarily need the momentum equation to solve the problem, you were just missing a piece.
Probably v/2 speed is optimal for wheel because it leaves water with no kinetic energy and incoming jet power is constant (assuming we have little energy loss on water turning inside bucket).
Your videos are awesome. What I don't understand in this one is that since both rubber balls when they hit the sensor come to rest at some point ( before the first bounce that is) this means that they both deccelerated to zero. since the non bouncy obviously took much less time to stop (it didn't have to wait for its deformation) this means that the force vs time must have been much higher and instant, like a Dirac function. So I am guessing something is wrong with the sensor, maybe a threshold or some kind of "inertia" that makes the sensor not responsive enough or a time resolution wasn't dense enough (although I have no idea how the setup works)
Thank you for giving us the history!
Radar guns use the redshift/blueshift of radio waves to measure the speed of cars, so I'm pretty sure you would also get a slight redshift with visible photons hitting a moving surface.
I don't think it's redshift. Redshift is the Doppler effect applied to light, i.e simply a geometrical consequence of the relative speeds of the photon and the observer (the wikipedia article on Doppler is good).
Here light is absorbed by the atoms changing their current energy state to a higher one, and then reemitted when they come back to a lower energy state. The energy states of the atoms are quantized (because the electrons jump from one definite "trajectory" to another, there are no "in between" trajectories), so they correspond to specific light frequencies according to the equation E = hf = hc / lambda where f and lambda are respectively the frequency and wavelength and h is Planck's constant. This is what you saw in your home made spectrophotometer: you lit the atoms with light and you got specific wavelengths corresponding to the changes of energy states of the atoms of the material. But they react probabilistically (because everything is probabilistical in the quantum world) only to wavelength corresponding to these changes of energy states. Had you lit the material with a laser with a different wavelength, you would have seen something else or maybe nothing at all (quite unlikely I suppose). So the atoms absorb light selectively.
When the photons are absorbed (from the incident direction) and reemitted (in all directions), they transfer their momentum p = h / lambda to the atoms thus creating light pressure.
I loved your experiment, BTW.
Interesting behavior of the Newton's cradle. I initially expected it to behave as it did, but your hypothesis made sense too. I was really surprised to see that the behavior was consistent with three and four balls. If you were to attach two of the balls, would it launch only one ball at a greater speed? I think the behavior seen in the experiment was caused by some very small delay between the impact of each ball.
Doesn't compton scattering play a role in it spinning? Is that how you figure out the photons relativistic mass? When I tried that I found the wavelength of the photon not the mass. What if compton scattering causes the green light photons to drop down to IR photons then they hit the black vanes and when they do the heat the black vanes heating the air and causing the black vanes to move along with photons hitting the black vanes directly?
Excellent video. Extremely informative. Personally I'd love to hear you go into relativistic mass...
As for the red shift assumption regarding the photons, I'd say that sounds right, but I am also not going to do the math there. ;)
Momentum happens because of mass, among other things. When we search for the source of mass in terms of weight, we turn to gravity. We say that gravity is due to mass, warping space. When we try to find mass, we find energies. So, mass/gravity is some sort of energy, analogous to constant acceleration, which is what a point on a rotating radius experiences, so, is gravity some constant acceleration?
Electronics makes classical mechanics more Interesting. Love it 👍🏼
This is very interesting. I have been wondering about the paper representation of what is going on with the spoon shaped part of the wheel. Because the illustration is represented as a 2 dimensional view and we live in 3 dimension in which situations do we need to us a 3 dimensional illustration to comprehend the problem.
I assume that water molecules bouncing off in a z axis doesn't affect the physics in this scenario?
My perspective is more focused on for example a physics engine point of view to simulate physical behavior to illustrate real life physics.
Another great video Ben! Would you mind turning the audio on your videos into mono in the future? Your camera pics up your voice and moves it from side to side ever so slightly. It's distracting when I'm listening with a headset. Thank you!
so if the Rotor is locked will the light bouncing off be brighter than when it is spinning at half the speed of light because the photons would stop? ( if the plates were curved)
@ninjaonaninja
Negative V just refers to the direction. The magnitude of the velocity of V and -V are the same, only the direction is different. Since velocity is a vector quantity it requires a direction. Hope that helps.
V/2 because the bucket and nozzle are acting like a differential, power is split evenly?
Would curving the fins of a radiometer make any discernible difference to its performance? Would it require a directed stream of energy, as with the water jet?
Isn't it all about the time the water has to do its work on the blade?! On the scoop shaped blade the water can push the blade for a longer duration since it is shaped so that it will catch the jet for a longer time, since the angle will change as the wheel turns? A straight blade will very quickly have an angle to the jet (or water fall) that will just make the jet glance off.
This is why on a water wheel the flow is directed over the wheel to fall on the far side of the wheel instead of just going under it or on the near side of the water fall.
More impeller blades are good!
I wonder if that is why anemometer (wind measuring device) are shaped like cup.
Yes, I was going to mention this, and ran out of time. Those devices are called anemometers, and can measure wind speed in any direction since the open end of the cup always "catches" more wind than the rounded end.
Forgive me if this has already been addressed. Regarding the pelton wheel, from a practical standpoint, if the wheel was moving at V, thus extracting no energy from the jet of water, the wheel would still be doing its job (crushing wheat or w/e). It would also be doing its job at maximum speed. Of course, after doing its job, the wheel would inevitably slow down, at which point the jet of water would collide with the cups, etc etc.. Am i correct? (possibly splitting hairs on this one, i understand lol)
The only way that the Pelton wheel can be moving at the jet speed (V) is if it is doing no work. At speed V, the wheel will have no torque, so it must be on frictionless bearings and not connected to a wheat grinder or generator. As soon as the wheel does work (grinding wheat, etc), it will slow down. Some electric motors have a similar torque curve, where they produce lots of torque at a standstill, but produce no torque at their free running speed. In fact, their running speed is determined by the point at which they have no torque! If they had more torque to supply, they would continue to turn faster until there was no more torque available.
Applied Science Heh, yeah i realized that a bit after making the comment. But what you mentioned about torque determining the running speed of an electric motor has given me something to think about. I'm working on a V.A.W.T. (vertical axis wind turbine) using an electric motor as the generator and torque is something I didn't have a full grasp on until tonight. Thanks Ben.
You would make one hell of a college professor.
the expanding air (due to heating of the black plates) pushes the black sides away, that's all. The vacuum is not perfect there or it would not work.
So when you changed the pressure around the Radiometer, you changed the light velocity. Would that mean the mass changed as well.
Did it not "optimize" at partial pressure?
How would variations in atm pressure effect a solar cells eff?
I have found that speed and mass and kinetic energy can be explained much like a container with water flowing out of it from the bottom. Speed/kinetic energy can be described as an amount of water and the container, it's mass. A % of the container being full, Say 50% correlates to 100mph, it takes much less energy/water to fill that mass to 50% of the mass is small, than of its a large mass. Once the container is full, you only need to keep up with the rate at which the energy leaks out. That is why it takes more energy to get up to speed, than to keep a speed. You have to fill the container.
Think about this, there are much more ways that this lines up.
I did this on my phone, it's not very clear. I'll repost later.
thanks helped a lot. im doing momentum right now in physics
Could you run a water wheel from city water and use the generated power to pump that water into a tank? Would there be any real value power generation wise if you did?
I imagine you would actually lose power if you did that. Why wouldn't you just pour the water into the tank straight from the tap? Adding the extra water-wheel/pump contraption seems like and unnecessary additional step.
wow you made me realize I can't really model a Newton's cradle.
Haha. I came up with the idea of a Pelton Wheel the other day. I was at an old mill and was looking at the blades and wondered why they didn't curve the ends in the shape of a scoop to make it more efficient. I guess they did.
"bouncy ball" is that the scientific term ;)?
Ben you could test this using your ruby laser in the vacuum bell jar... that would be awesome...
If photons always travel at the same speed, why would they be red-shifted? Surely that is a function of wave-length, which wouldn't be changed. If you shine, say blue light on the radiometer. will it rotate at the same speed as red light?
Why can´t I like this more than once?
Is there a water wheel vs Pelton wheel equivalent for air/gas?
1:25 : but a rotation is not a vector so why it will apply to rotating perles/balls who hit others rotating perle/balls ?
Question. Which of these two scenarios has the higher kinetic energy? Two cars colliding head on at 60 mph each or one car hitting an immovable object like a bridge abutment at 60 mph?
REPLY
why if you shine red spectrum light it does not work?
Really grelt video. How abort making Real physics and electronics videos with theory and application which You do fantastic. Love your videos thanks
So the red line is like measuring Horse Power but with some other unit but the same way to measure force? My brain is hurting trying to figure out an analogy/equivalent to what the red line is. I have an analog scope (don't know much about them fancy pants new fangled dig scopes) so the red line is calculated by some math function in the scope? If you had units for the red line what would it be newtons/time? or newtons/length?
Wait so is the velocity squared the dot product of the velocity with itself?
yes
I sometimes work with high power photo flashes. But if I hang a sheet of metal (1,5 mm aluminium) in front of it and trigger the flash, the sheet will "chime" just by the light.
I have always been wondering if it's the photons bouncing of like your video or if it is a thermal phenomena due to the high energy released from the flash tube.
Any thoughts?
Would it work better with mirrors?
really good explanation, thank you sir!
How would you get the Pelton wheel to rotate at v/2?
I understand why it is the best operating speed, but how would you keep the wheel spinning at v/2? I could think of adding a damper to the system, but then you're just bleeding energy, and then is it still as efficient?
Or will the wheel naturally spin at v/2 because that's when it gets the most force (change in momentum)?
Is it something like if you plot input water speed versus wheel speed (assuming they're along the same direction), is there some sort of stable maximum/minimum?
Probably by adjusting the nozzle diameter.
h0lx Yeah I was thinking about that, but then you'd need a controller (or something) to sense the velocity of both the wheel and the incident flow and be able to actuate the nozzle. I guess you could do it by hand and approximate it, I'm just not sure how exactly it's done.
Also now thinking about it, you could also hook it up to a gearbox, that way all the damping will do work.
Ninjaplz10154 by mathematical models
onyzee Yeah, so there's no natural mechanism that keeps it rotating at v/2, so it seems like a waste of energy
this is cool. More physics please.
I agree on the redshift.
Great video !
Awesome video
WELL, THIS MAY BE DUMB BUT.... I think there are air molecules above the vanes. The dark vanes warm the air. The warm molecules fly away from the vanes exerting force which causes the vanes to move away.
While I was looking at your drawing of the water entering the scoops it occurred to me that the scoops would enter and leave the stream without being in the optimal position for very long. What would the best arrangement be to make a water wheel like that in real life? 6 cups and 5 jets to stagger the forces? Perhaps you could make something simple to show us.
I think most commercial Pelton wheels have many more buckets, and often more than one water jet. There are some photos here: en.wikipedia.org/wiki/Pelton_wheel
When a boxer punches is he or she transferring kinetic force or momentum force?
Are those pressure sensors, peizo sensors?
If I want to break open a door with a baseball bat, should I use a heavier bat ( more momentum) or a standard bat but swing it faster ( velocity squared and more kinetic energy) , what would be more effective?
So a rubber mallet has more force then a dead blow??
Great video! (There's a small typo in the decription I think, wouldn't photons impaCt the rotor? :)
No
Is a Red shift lengthening or shortening the wave length of the light? and would a magnifying glass work as a nosle?
Hi, a shorter wave length = higher frequency = more energy. This means a red shift is a lengthening because the wave loses energy. Since the speed of light is constant within a medium it has to lose energy that way.
A magnifying glass however would itself not act as a nozzle because the light is not only shifted in color but has also changed its speed. The speed of light depends on the refractive index of a medium! This means the total energy for the light stays the same within the glass and it leaves the glass as it came in.
If red light comes in, it is blue shifted and slowed down within the glass. Leaving the glass it is sped up and red shifted again.
There not such a literal analogy for the nosle as the light does not need to be placed on a specific part of the radiometer vane. The amount of energy transferred depends on the reflectivity of the radiometer vane which is why this is different on either side so that more force is applied to one side than the other and so it will spin.
The analogy is between the cup changing the direction of the water by directing the flow to the opposite direction and the reflectivity of the radiometer vanes changing the direction of the light by reflecting it.
A little over 7 minutes in you blew my fucking mind. I never once considered that sound is energy. I mean - if someone asked me that before I'd say yes, I just wouldn't put 2 and 2 together in the bouncy ball experiment you did.
How can you measure the force without extracting any energy?
redshift = longer wavelength?
Brain explosion. Now I need to pick-up the pinky parts of grey matter lying on the floor.
When you form the kinetic energy from the velocity, you aren't really "squaring the velocity vector" -- you are taking the scalar product of the velocity vector with itself to obtain the square of its speed. Us mechanician's like to be very precise with these things.
Very interesting. Thank you!
My radiometer works from the radiant heat of my gas fire, I assume it's the heat and not the light. It would be interesting to see if it works from the heat of a hot plate that doesn't emit light.
It emits a lot of infrared radiation, which is almost the same as light, but you can't see it with your eyes.
Where does the kinetic energy go?
If there is an 80kg target that completely absorbs the .025 kg bullet moving 300 m/s the momentum equation gives the target about 0.1 m/s velocity and .4 joules kinetic energy.
What happens to the 1000+ joules of energy the projectile had on impact (assuming target absorbed it and the bullet didnt continue traveling past target)? Please help this make more sense - thank you
All the energy in this case is used to bring the bullet to a stop. If the target did actually move, the bullet would still have some momentum forward (thus it would be moving forward). If the bullet was elastic, then i would store the energy and bounce. Its important to consider whether a collision is elastic or not, before using the momentum equation.
Dangwell-explained to this guy! Thanks!
Very useful
Great video. I just reviewed MIT OCW 8.01 Lec 15,16,17 by Walter Lewin just the other day. This is an awesome application. May try and work through it tomorrow or soon.
You may have been the last person to view that lecture. All the online resources have all been removed indefinitely due to a sexual harassment investigation: newsoffice.mit.edu/2014/lewin-courses-removed-1208
Applied Science
Wow. That is unfortunate.
Good thing I downloaded the full 8.01, 8.02 ,8.03 series a couple years ago.
Applied Science
Man, i just searched youtube and the opencourseware 8.01 series was there, I hit refresh a couple min later and it was gone. :(
Other people have the series up still... probably until MIT makes them take it doen
Applied Science OMG no no no no no no
Given the time in history, without the aid of modern measurement equipment, Newton was a true genius.
Photons don't have any kinetic energy since they don't have mass, right?
lol
I'm certain that the Doppler shift is the answer. it just has to be. Well that and gravitational waves.
This is why spoons are much better at mixing sugar in your coffee than "mixing stabs" do, especially round ones