8.01x - Lect 26 - Elasticity, Young's Modulus
Vložit
- čas přidán 6. 08. 2024
- Elasticity - Young's Modulus - Dramatic Demo
Lecture Notes, Elasticity of Metals: freepdfhosting.com/f7dd12629c.pdf
Assignments Lecture 25, 26, 27 and 28: freepdfhosting.com/03ca75eadf.pdf
Solutions Lecture 25, 26, 27 and 28: freepdfhosting.com/f15cd35103.pdf - Věda a technologie
I have to stop staying up half the night watching Dr. Lewin's lectures! This is the fantastic part about the internet.
I understand ya😔✊
SAME!
Hopefully I can learn some physics from all of this insomniac behavior I have taken to...
the experiment was ingenious..kept me in suspense all throughout, you have done a marvelous job...love you for that
:)
Very good explanation of Hook's law and Young's modulus. I recommend this class for every mechanic, to get better understanding why we use torque wrench for key thread joints.
Overtightening screws can demage them permanently.
I like these lectures very much.
I keep watching your (essential) videos even after passing my physics exam. You made me love physics! Thank you very very much Dr. Watler Lewin
Watching the stress strain test I am reminded that boats are tied to their mooring with nylon rope. It's elasticity helps to keep a boat from tearing itself to pieces during a storm. In contrast lines used to control sails are often made of a Kevlar composite (low elasticity) to take full advantage of wind gusts and push the boat forward with maximum efficiency.
I sometimes wonder where you get these demonstration ideas from! Truly Amazing!
Beautiful and conceptual lecture. Fascinating!
THAT WAS AMAZING LECTURE ABOUT ELASTIC MODULUS BIG THANKS PROFESSOR U R THE BEST GAVE US THE BIG PICTURE OF THE STRENGTH OF MATERIALS
I'm a Sri Lankan Physics student. Sometimes I can't understand this theory. And I want to thak u Dr.
This is genius lecture I ever seen
Wonderful lecture and demonstration!
A brilliant professor!! Thanks a lot sir.
This was really boring when taught by my teacher, but surprisingly interesting when Mr.Lewin teaches this
Hii sir I belong to India and I am a Big fan of yours .........Because if everyone can explain as so much as than PHYSICS WILL BE SO EASIER FOR AL STUDENTS .......THANKS FOR THAT. ...SIR
Most relevant channel name
I EVER SEE ❤
Nothing in this life beats starting your sunday whit some good coffee and a lecture from Walter Lewin :)
:)
Hello sir @walter lewin,
Where do I get your lecture on surface tension?
Awesome lectures
Excellent lecture Sir. Thanks and Regards 🙏🙏🙏🙏🙏🙏🙏🙏
Amazing. These actually made me love physics.
When I study, My teacher just show me only Ymodulus formula, let me read only text example, then exam.
Since I found this, and all of Dr.lewin lecture. I understand them all clearly. Thanks you very much for sharing us. From Thailand.
That's great!
At 19:30 doesn't the mirror in the experiment affect the extension of the wire?
Prof. Lewin, I have a doubt. Initially, when you assumed the two rods to be joined to each other (in series, as they say), wouldn't the upper rod experience a force equal to Applied Weight + Weight of the rod below it. Therefore, shouldn't the extension on the upper rod be more?
question unclear
@@lecturesbywalterlewin.they9259 Prof, at the start we studied the case of one rod over the other, i.e. in series. There, a force was applied at one end of the rod. You said that the stress would be same on both rods as force on both was same.
Here is my doubt. Why didn't we take into account the extra force that is acting on the upper rod due to weight of lower rod?
@@mehulpandita8852 we can do it either horizontally, also, the force due to rod is negligible as compared to the force applied
@@mehulpandita8852Static equilibrium point must be reached for tensions to even out between upper and lower parts of the system.
If the tension wave is at the beginning of the motion, there will be a difference in tension between the upper and lower parts of the system, with the lower part free-falling initially (undergoing a forced motion) whilst the upper part is fixed.
Next, the lower part goes into equilibrium with motion at constant speed whilst the upper part accelerates because the tension wave has reached it (experiences a force).
Finally, both parts end up moving with the same speed and both tensions are the same.
Force and tension concepts are subtly different, one being dynamic whilst the other is relatively static. Same idea goes with gravity and weight.
Hello professor! I'd like to ask, are young's modulus and thermal expansion coefficient somehow related?
1:13 What happens if the springs that are in series are not yet in static equilibrium and the applied force on the end is a mass that is freefalling under gravity?
Please depict as tensile force vs time charts.
(just underneath each spring; BM=Before mass added to spring system, BE=Before equilibrium, AE=After equilibrium).
wow this playlist is great for jee prep as well
4:10 u said that it is equivalent to two rods attached parallel and when F is applied only half delta l is the extension then why can't we take same reason in first one and say that it is like two rods of areas a/2 attached parallel so that it will also extend half delta l only not delta l?
Hello. Professor didn't mention Yield point. Are the Elastic limit on the curve and yield point same? Also there is something as upper and lower yield point. Thanks.
en.wikipedia.org/wiki/Yield_%28engineering%29
But why the rod gets that specific shape i mean why it gets semi spherical shape from its sides
at 49:00 when explaining the reason why the upper string doesn´t break, wouldn´t the same reasoning apply to the lower string? Because we aren´t giving the lower string enough time to reach delta either.
I did not explain during my lecture this last demo. It was up to my students and now up to my viewers to give the correct explanation.
@@lecturesbywalterlewin.they9259 Alright, thanks
Thank you.
Thanks from India
@@lecturesbywalterlewin.they9259 Thankyou sir for your reply but i do not think i was able to put my question forward
My question is
How is it possible that the copper wire was following hooke's law or linear graph but wasn't able to show elastic behaviour as we see the wire was having a permanent set (wasnt able to completely return to orignal state after taking off load)
As till following hooke's law it should show elastic behavior i.e return back to its original state
Kindly help Sir!!
Hatts off from India❤️
Watching previous lectures I was expecting Prof. Lewin to actually walk the rope:)
ha ha ha
How change in length is directly proportional actual length when taken in series, could any one explain?
You refer to 1:13.
∆l/l=total(∆l)/ total(l), where "total" is over all the springs that are in series. Also, the springs should be in static equilibrium.
i have seen the experiment on universal tensile machine but that was boring but the method in this class was more fun and visible due to magnification...thank you
:)
47:28 The block somehow reminds me of a resistor...
Is electric current really due to some type of tensile stress waves?
how did you derive that last equation relating delta(l) to delta(t)^2
how many minutes into the lecture?
49:03
>>>>delta(l) to delta(t)^2>>>
x(t) - x_o = v_o*t + 0.5 a t^2
v_o=0.
I don't know why, I get energies when I watch your lecture.
Sir have you video lecture on other modulus like bulk modulus
I don't think so
Speaking of bulk modulus, the speed of sound formula really depends on bulk modulus rather than Young's modulus.
There are three moduli values, that are all inter-dependent for materials whose response to stress uniformly in all directions (i.e. isotropic materials). The Young's modulus (E), the shear modulus (G), and the bulk modulus (B). There is also the parameter called Poisson's ratio (nu) that applies. From any two of the above 4 values, you can calculate the remaining two.
thank you professor
You are very welcome
At 20:27 why does ∆theta = ∆l/d?
Ahh I had this problem too, but I think I figured it out -
I think he is doing a small angle approximation. The small angle approximation for sin(x) = x, so as sin(Δθ)=Δl/d, and sin(Δθ)=θ, Δθ is therefore equal to Δl/d
Sir is the spring showing simple harmonic motion continuously or for a particular period of time ? and if it is showing simple harmonic motion continuously why can't we see it on scale?
use google or watch my lectures on springs and SHO. Also my 8.03 lectures which include damping.
Hello professor... thanks to provide these precious materials... I've a question... many physicsist till today don't accept string theory to be true...what's about your openion?
string theory hasa made some impressive contributions. I suggest you read abour it. en.wikipedia.org/wiki/String_theory
wow. great experiment
Thank you 🤗
24:39 "tja!" One always retains the native tongue.
Also at 25:21 "ja ja"
THAT WAS A SUPERB LECTURE PROFESSOR...... I
BUT A TINY DOUBT...
WHAT WOULD BE THE CONSEQUENCE IF ..
F=K/L^2 AND NOT K/L
ask google
Sir Walter Lewin, When you demonstrate the 2100 Hz frequency of sound using the metal rod. you rub the rod with your fingers after you hit it, Why did you do so? 46:37
good question - I watched my video from 46:00 - 47:00 By moving my finger softly along the rod I suppress any possible frequencies which are not longitudinal.
I have wondered the following for years.
If I keep my fingers a millimeter apart, I can see a cigar floating between my fingers against the light.
I can also play with the size of the object.
Usually google is my great professor, but he also has no answer.
Sir, I am doing your assignments for 8.01 and I´m having trouble understanding your solution for problem 10.5 Assignment 10.
In your solution, you only take into account one half of the meterstick, but as I see it, the other half is exerting a force in the middle of the stick equal in magnitude but in the opposite direction, so I end up having twice the force you calculated. Why do you ignore the force due to the other half??
soln is correct as you are asked to calculate the tension at the *center* of the stick
@@lecturesbywalterlewin.they9259 So, you could just remove one half and end up having the same tension??
Sir, in the stress-strain curve: is stress dependent on strain? I mean why is strain on the X-axis and stress on the Y-axis?
Google "stress" and google "strain".
Stress = Young's modulus x strain
compare it with y = mx + c
young's modulus is the slope.
When you plot what happens after reaching the ultimate stress, strain continues to grow while the material breaks. There will be a reduction in the load it supports as it is in the process of breaking. This is the case for a ductile material, but not for a brittle material. Brittle materials rupture immediately after reaching the ultimate stress.
Because of this, in order for stress vs strain to be a function, you end up having to put strain on the x-axis. Otherwise you have two different strain values, for a small range of stress values.
During the experiment, doesn´t the rod that´s attached to the mirror affect the outcome?
yes it amplifies the displacement of the laser spot on the wall.
@@lecturesbywalterlewin.they9259 Is it something we should take into account during the experiment ?
watch the lecture - do your own geometry - Make the end of the cable go up by 3 mm then calculate the angular change of the mirror and take it from there
At 6:08 which website were you talking about sir ?
It was related to the topic that I discussed.
Doesn’t the environment temp also vary the break
If the environment is 3000K the wires melt
Sir, can I ask you a question ? If two ropes are hanged from the ceiling , the second is twise as long as the first , if 20 kg mass will stretch the first one by 1 mm , by how much it will strech the second ?
+Фарид Гасанов You should be able to answer this on your own.
+Lectures by Walter Lewin. They will make you ♥ Physics. I answered by 2 mm, is it correct ? I thought that rigidity coefficient is inversely proportional to initial length.
+Фарид Гасанов yes, of course. If length L gets 1 mm longer, then length 2L will get twice as long
32:26 it's so satisfying 😍😋
Sir we are taught that vector division isn't possible ...so how force(a vector ) divided by area (a vector) is coming out as a scalar ( i.e. pressure )? moreover why force per unit area is known as stress but not pressure over here? aren't they same?
that is a good question, and I have wondered about that. Essentially, at the most basic level you treat Force and area more like scalars. If you have a scenario where they do not act at right angles, then you take the component that is parallel to the normal of the area. Actually, technically speaking, area isn't so much of a vector. We often find it useful to define a vector that is normal to the surface that you are dealing with, and had a magnitude that is proportional to the area itself, which is a scalar. In short, you take the magnitudes of the force and area, sort of like how you do a projection of one vector onto another, and you divide their magnitudes, which is perfectly allowed.
NB pressure can technically have a negative value, so it is I would argue very similar to how flux works- but flux is the dot product of two vectors. I hope that helps- that is at least how I understand it :)
how is area a vector? cross-sectional area is always a scalar.
@@MultiCoolman125calculate the Electric flux through a surface: it's the integral of the dot product E dot dA, dA is a small surface element, it is a vector!
@@lecturesbywalterlewin.they9259 Well, in that case, it is a vector. Area is a vector when there needs to be a sense of direction like to determine electric flux, since it makes sense for electric flux to have a direction, going in or out. In cases of the Young's modulus equation, area has to be a scalar, because you can't divide two vectors. Also, in this equation, there's no reason for area to be a vector. There doesn't need to be a direction as with electric flux.
Hello sir , I am engineering student and want to study" Strength of material" more deeply so which book should I read .
I do not know
I used "Mechanics of Materials" by Ansel C Ugural.
@@lecturesbywalterlewin.they9259 hello sir you have been greeted
Kindly respond if anyone knows🙏
Respected sir
I love the way you teach but am facing a doubt that when the graph was linear the wire didnt return to its original length (didn't show elastic behavior)
Why so? Why was there permanent deformation sir? Shouldn't the wire be elastic when in the linear part of the graph or Following hooke's law
Please help sir m really confused
after a cable breaks it will never go back to it's original length
from 20:31 .... delta theta=delta l /d .... is sin delta theta=delta l /d ..... or im missing something?
Both of these are small angle approximations. In radians, theta is approximately equal to both sine of theta and tangent of theta, in the limit as theta approaches zero.
Sir, will the Young’s modulus of a material change if the length or breadth is altered? I’ve asked this doubt to literally every professor in my college and none of them gave me a rational answer.
young's modulus is the ratio of stress to strain. if you change the area, you change the stress. If you change the length, you will change the strain. so yes, young's modulus will change if you change the area or length
Young modulus only depends on the material of the wire. It will not change. With lower A, increase in length delta-l will be higher, so Y remains the same. Y is a constant , which comes directly from Hoke's law.
Given that the material is linear-elastic, Young's modulus only depends on the identity and conditions of the material. It doesn't depend on geometry or load. One particular "condition of the material" is temperature, which if hot enough, will cause a steel beam to bend like a wet noodle while still solid, and be useless as a beam.
At time 44:24
Young's modulus is very low material is more elastic why sir?
It depends on how you define the adjective elastic, because people and books are inconsistent by what they mean when using "elastic" to describe what Young's modulus implies.
Better adjectives are stiff and flexible. The lower Young's modulus, the more flexible the material. The higher Young's modulus, the stiffer the material.
You tell that the horizontal portion is called the "plastic flow" but at 13:50 you mark some other part of the curve as "plastic flow".........WHY?
i think the plastic flow defines the phase that comes after our material starts to lose his elastic properties thus the graph stress-strain is no longer linear
use google
the way i defined plastic flow is wrong/flawed? D= and thank you for taking the time to respond to our questions :o
both is correct
RESPECTED SIR , THE LINK IN THE DESCRIPTION BOX IS NOT WORKING PLS HELP !! 🥺🥺 FOR THE ASSIGNMENT AND NOTES
use the playlist "8.01 Homework, Solutions, Exams and Notes"
Lectures by Walter Lewin. They will make you ♥ Physics.Under what conditions does Hooke's law does not hold,professor?
when k (in F=-kx) is not constant
And when or how is that possible?heating the metal?
one way is to overstress a spring
Sir if we have two springs with different k and they are parallel to each other and some force F acts on them (2:00) will the ratio of forces that act on each spring be any different than when the k is equal (when k is equal the ratio is 1:2, so force that acts on each spring is F/2)
I cover spring combinations in my 8.01 lectures and in my 8.01 Help Sessions or use google
Thank you so much professor :D
@@five5059 For springs in parallel, the k-values add up directly. For springs in series, the k-values add up in reciprocal, in a similar formula as the way capacitors in series add up.
This assumes that the springs are arranged in a way that they don't apply a different torque to the object at the end. Such as co-axial springs of different k-values in parallel.
The reason the Young’s modulus (7 vs 11) was so far off is that you need to use the the change in force that resulted in the delta L not the total accumulated force that has been added. Neat demonstration
I shouted when the wire broke like a football fan or something waiting patiently for the goal XD
What is the average age of those students?
18.5
Why r they studying 11th and 12th again 😅
Why does the lecturer who is obviously in USA, use meter, kg and so on instead of feet, pounds and so on?
Ghl Scitel it's always preferable to use the SI system of units which is universally accepted. No matter which country you live in
Because this is physics. Physics is taught in the SI system even in the USA
He answers this question in the first lecture. He acknowledges the system of units consisting of ounces/pounds, feet/inches/yards/miles, and expresses his opinion about it being an uncivilized unit system, due to conversion factors that are all over the place and challenging to remember.
Physics formulas also only work when you use a consistent system of units, which the SI system is set up to do. With the exception of kilograms that generally need the kilo part of the unit, most SI units in their base form are a consistent set of units. The same cannot be said for the US customary system. In Engineering classes, they teach you to solve problems in both systems. In Physics classes, it is generally seen as a nuisance to have to work with US customary units, and they are usually avoided altogether.
They have to use a world standart measur because there is people of all the world😔✊
💜🥰🇳🇵
You done a great job but stil 7.7 x 10¹⁰ 😓
I want to read a vortex flow
These students are so boaring.. Whole class is unresponsive... The teacher is so amazing... But why in india we learn all these in high school.. While in US they learn in university...