MIT Physics Demo -- Centrifugal versus Centripetal Motion
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- čas přidán 5. 11. 2008
- A wooden ball is attached to the rim of a spinning wheel. The ball is held in place by a string. When the spring is cut, the ball flies in a straight tangent to the wheel.
In the camera's frame of reference, the ball constantly accelerates around in a circle due to the centripetal force pulling it inwards. When the string is cut, the acceleration stops, and the ball flies away in a straight tangential line. When the string is cut in the rotating frame of reference, a ficticious force (centrifugal force) accelerates the ball.
See the original video on MIT TechTV - techtv.mit.edu/videos/740-mit-...
Woah! Nice catch there wild man!
While the ball is spinning at a constant angular velocity, the axle must exert a centripetal force on the ball to keep it in place. When the ball is released, this force vanishes. That is the only thing that resembles "recoil". No force is applied to the ball to release it, rather a force preventing it from flying off is taken away.
The point of the demo is to show why centrifugal force is a fictitious force (it is only needed when you are in a non-inertial reference frame). Here's how it works: suppose you think that centrifugal force *is* real. Then which way is this force pointing ? Presumably you would say it points outward (radially outward from the center of the circle). But if this is true, then at the point that he cuts the string, the outward force should move (accelerate) the ball in the outward direction (away from the center of the disk). But it does not do that. The ball moves in the direction *tangent* to the disk (since the ball is moving straight up at the moment the string is cut, it continues straight upward, *not* radially outward). This proves that there is no outward (centrifugal) force acting on the ball. The only force acting on the ball (apart from gravity) is the tension in the string. This is a centripetal force that points towards the center of the spinning disk. Without this force, the ball wants to move in a straight line tangent to the edge of the disk. But the string (before it's cut) keeps pulling it toward the center as the disk spins around, changing the ball's velocity to keep pointing tangent to the disk.
Another way to think of centrifugal force is this: Imagine you are on one of those cylindrical rides that spin around and the floor drops out, leaving you pinned to the wall behind you (or equivalently, that you are on a merry-go-round). Your body "feels" an outward force that's pushing you towards the wall (away from the center). But you are *in* the non-inertial (=accelerating, since constant circular motion is caused by acceleration towards the center) reference frame. Someone outside this ride could potentially see what's really going on: It's not that there's a force pushing you towards the wall, it's that the wall is spinning around (accelerating *towards* the center), while your body just wants to keep going in a straight line tangent to the rotation. But the wall keeps moving you along with it, making it feel like there is a force pushing you against the wall (really it's just the wall carrying you around in the circle). In effect, you are the ball on the string here.
thanks MIT, we can always count on you to further our knowledge. this video is proof that your university is the best
I love MIT and I feel lucky in a good way to have access to these videos.
After watching this particular one, I think I finally understand the centripetal vs centrifugal forces.
In short, centrifugal force is simply inertia. :)
If you follow the spot on the disk from which the ball was released you will find the ball does indeed move vertically outward from it, for a few degrees anyway. Try mounting a camera to the wheel and film from the wheels perspective.
nice catch at the end.
They're trying to demonstrate rotation is actually applied in a straight line. Centrifugal and centripetal forces are equal while the ball is traveling in a circle.
Well, its a virtual force. It depends on your frame of reference. Say you're in a car turning a tight corner. In the frame of the car, you appear to feel a force outwards, but in the static frame the car feels a force inwards and essentially hits you.
I know you get it, just thought I'd point it out for others :P
Centrifugal (center-fleeing) is fictitious and centripetal (center-seeking) is real, but this choice depends on reference frame. In the stationary frame the ball goes off tangentially and the centrifugal force is clearly fictitious. Sitting on the disk looking at the ball, it appears to move directly away at first, giving the illusion of a center-fleeing acceleration--the effect of the false centrifugal force. The centripetal force is quite real--it is what keeps the ball moving in a circle.
Of course, in the comment below I begin with a typo by writing centripetal when I meant to write centrifugal - ha ha ha.
you could have colored the ball with a bright colour to increase its visibility
So would I be correct in saying that the centripetal force is the force being exerted on the string (between the center of rotation and the ball) and the the centrifugal force is the tendency of the ball to leave it's orbit on a tangential line?
Great video by the way. I had always seen these two terms as synonymous.
@Oshyrath thanks
You misquote me. Read again. Never said 2 forces act on ball nor on string. I even put By and ON in caps!
According to the first Law of Newton, If an object is at rest, it stays at rest and if it's in motion, it remains in motion on a straight path with a constant velocity, unless there is a net force acting on it. You say there are 2 forces acting on it that are equal in magnitude but opposite directions.
If it was so, net force would be zero and the ball should have followed a straight path. Since it is doing circular motion, it makes sense to me to have only one force which is centripetal force
I learned nothing from this.
If you want to talk about centrifugal force then mount your video camera on the wheel.
Wrong. They string pulls in on the ball with centripetal force. The ball pulls out on the string with centrifugal force. The only thing fictitious about centrifugal force is that it does not act ON the ball. It is exerted BY the ball. When the string is cut both forces disappear. The ball moves off on a tangent because the centripetal force on it disappears. The string slackens because the centrifugal force on it disappears.
is there centrifugal force due to momentum?
Many have said there is no such thing as centripetal force. They've invoked reference frames, Newton's 2nd law, and quotes from authority. They've also focused on forces acting ON the ball. I now invoke the Newton's 3rd law. Since the string pulls IN ON the ball the ball pulls OUT ON the string. This outward force is CENTRIFUGAL. It's NOT exerted ON the ball - it's exerted BY the ball. Now before you knee-jerk your way to foolishness read again. A pulls IN ON B whence B pulls OUT ON A.
@thefencejumperengine
I imagine you can test this yourself. If you've ever swung a ball (or any weight) on a string when you were little and it let go, you can probably remember what happened. :)
What has this taught the student.
Well as we can see he certainly learned that gravity has an effect on the ball.
My proof!
Study the first time the ball is launched, the student overlooks the effect of gravity and observes the ball fall, without trying to catch it
After the student has observed the trajectory of the ball based on it's speed / trajectory vs gravity he can therefore acccuratly predict where the ball will go and as a result catches it.
Science at work here folks
xD
this isnt centripetal motion. centripetal motion means "motion toward the center"
MIT students, capable of doing anything.....except how to de-interlace a youtube video.
When you swing a ball on a string the ball exerts a centrifugal force on your hand which you will feel thus it is real.
Frank,
They say centrifugal force isn't 'real'. Of course it's 'real' as you pointed out, it's effects can be observed and felt. I think it would be better understood as a 'secondary force'. A result of energy applied to axial rotation/angular momentum.
@@paulstovall3777 I think the point of the demo is to show why centrifugal force is a fictitious force (it is only needed when you are in a non-inertial reference frame). Here's how it works: suppose you think that centrifugal force *is* real. Then which way is this force pointing ? Presumably you would say it points outward (radially outward from the center of the circle). But if this is true, then at the point that he cuts the string, the outward force should move (accelerate) the ball in the outward direction (away from the center of the disk). But it does not do that. The ball moves in the direction *tangent* to the disk (since the ball is moving straight up at the moment the string is cut, it continues straight upward). This proves that there is no outward (centrifugal) force acting on the ball.
@@MH-oc4de
Of course because restriction to maintain that angle of momentum has been released, ergo its' direct link to centripetal 'force' (neither of which exists without the other). Maintained, however, and you get mechanisms such as the 'gravity wheel' used at fairgrounds. Under those circumstances, these 'forces' are quite 'real' and can be positively used.
@@paulstovall3777 That's not correct. Centripetal force is real and exists without centrifugal force, which is not real and is simply used as an accounting trick. To understand this, you have to be very precise about what is generating the force. For instance, it's easy to see that both gravity and the tension in a string are capable of generating (inward directed = centripetal) force on a body. When you swing a ball tied to a string around in a circle, how do you imagine an outward ("centrifugal") force is being applied to the ball ? What force acts outward ? How ? There is no real force that does that.
@@MH-oc4de
Acceleration.
There is only one force and that is not centrifugal force..
It's toward the center. To see it, all you need to do is to define 2 velocity vectors and call them as initial and final velocities. Then apply the acceleration formula a=vf-vi/time.
Time is a scalar quantity. The only quantity that has direction is velocity. So subtract velocities and you'll have the direction of the acceleration which is towards the center.
@fredb3
Degree level at that xD
I also love MIT videos but this particular one is not reliable.
Maybe in your next MIT demo, you can explain how white lab coats make you appear smarter than you really are.
There is no such a thing as centrifugal force.
second comment. yo trabajo en esto para inventar mi impulsor inercial
first comment! i love this channel
Ridiculous video
well this was pointless and useless.