If you hang a pendulum in a vacuum, then shake it. would the pendulum move forever?

Question

Thanks for answering.

Please explain, why?

Answer 1

No. It will not move forever because you did not eliminate the resistance completely. The remaining resistance is still in the pivot joint of the pendulum. It will not matter how you lubricate it or use the best bearing, There will be always resistance to its movement and this resistance will convert the stored energy of the pendulum into heat in a slow rate.

Editon:

In adding to this comment, Most answer think that gravity will eventually stop the pendulum which is wrong. Gravitation aplies to the pendulum is the same with acceleration of falling bodies.

A sattelite will keep falling around the curvature of earth if the friction is totally zero.

A bullet will decelerate until it reach the highest point and start to accelerate going down having the same velocity at any given point when it was on the way up.

Now you can combine this two phenomenae into the pendulum which is also going up and decelerating until it reach the highest point. Then it will start to fall down accelerating in the same velocity at any given point and the fall is in orbit around the pendulum's pivot point.

Then in this case, if the friction is totally eliminated, (Which is impossible on pendulum's pivot bearring,) Ideally, it should continue forever.

Answer 2

I must make the disclaimer that this is a guess based on what little I do know of physics.

No. Gravity pulls harder on the pendulum each time it swings past the center than it does when it is towards the side and this eventually eats up the initial motions energy. The entire mass of the pendulum is in a direct line up and down at the center, which might explain the additional effect of gravity. The pivot seems it would also be a factor as the weight on the pivot is in constant motion oscillating between pulling sideways and pulling downwards.


HOURS LATER: I thought about it and Jon S is right, the friction is what would stop it. Give him best answer.

Answer 3

If you hang a pendulum in a vacuum and get it going AND it's in the precense of a gravitational field, it will not keep going forever but not because gravity stops it. The force of gravity is what is called a "conservative force". When the pendulum is as high as it will go, it has maximum potential energy and 0 kinetic energy. When it's as low as it will go (and moving the fastest) it will have 0 potential energy and maximum kinetic energy. When the pendulum is swinging in a frictionless system, one form of energy is simply converted to another.

But friction isn't a conservative force. When energy is put into friction it is dissipated in the form of heat. When you rub your hands together to warm them some of the energy put into moving your hands is converted to heat, and thus lost.

The energy the pendulum uses will be slowly dissipated by friction. This could be friction at any joints or friction within the string itself caused by the interwoven fibers of it, or it will be lost in other internal forces within the string/joints.

IF you had a truely frictionless pendulum system (which just doesn't exist) then it would swing back and forth forver.

Answer 4

"asimovll" is correct.

Just removing air resistance, you have not eliminated all frictional forces....friction will still exist at the pendulum's pivot point, eventually slowing it to a stop.

Of the pivot point was frictionless (impossible), then it would continue swinging forever as long as it is within a gravitationall field.
Gravity in and of itself is not a dampening force, it is the force that keeps it moving in the first place. Some initial value of the pendulum's Kinetic energy and gravitationa Potential energy keep it moving back and forth. Gravity only works to convert these energies back and forth. However, due to the 2nd law of thermodynamics, no energy conversion is 100% efficient, so eventually, after a long enough period of time, the energ would all be lost to heat.

Answer 5

If it is on a planet in a vacuum, the down pull of gravity would eventually pull it down to a stop just like earth normal.
The lack of air resistance might let it swing a little longer though.
On a planet with less then earth gravity it might swing longer.
On a more then earth grav., it might swing less as the pull down should stop it faster.

After rereading the other answers, I think robin* has the best and simplest answer so far.

Answer 6

Energy can never be lost, so Kinetic energy cannot be lost, so the pendulum will keep swinging. If that is not true, then you cannot explain a comet's orbit, it gets nearer to the star and goes back to its original position. So there!

Answer 7

No, even in a vacuum a pendulum won't work without gravity, and with gravity it will eventually be pulled to a standstill.

Answer 8

removing air or bringing it in vacuum should not really make a huge difference as the motion of the pendulum would and does depends on physical factors like gravity,N force,tension on the string,etc.so .....

Answer 9

No! Although there would be less resistance due to a lack of atmosphere, there still would be gravity pulling on it and it would stop. The energy that you initialize by the shaking would be lost to gravity.

Answer 10

No' Because it the pendulum would not have a constant sorce of energy or foce behind it.