# The effects of gravity on a pendulum

1. Oct 18, 2006

### onestarburns

Forgive me if this is anything but trivial, but my physics teacher said in his lecture the other day that gravity has nothing to do with a pendulum coming to rest? how could this be? according to him, the reason for the pendulum coming to rest is air, but when i inquired about puting one in a vacum, he said that friction would then play a role. am i wrong to feel that this may be false? and if my physics instructer is correct, could you help me understand why gravity plays no role in the object coming to rest. thanks
james

2. Oct 18, 2006

### brewnog

He's right.

Gravity plays no part in the pendulum coming to rest because no energy is lost to it.

If you lift a weight up to the top of a hill, and then let it drop, the amount of potential energy you give it by lifting it up is exactly the same as the amount of energy it loses whilst being returned to the bottom.

The same principle applies to a pendulum; once it reaches the bottom of its stroke, all the energy it had at the top has been converted to kinetic energy, and could thus (ideally) carry on back to the top of its stroke again.

Your teacher is correct in stating that the forces acting against its movement are friction (in the bearing mechanism) and air resistance.

3. Oct 18, 2006

### HallsofIvy

Your teacher is absolutely right. Gravity has everything to do with where the pendulum comes to rest but nothing to do with the fact that it does come to rest. Consider what would happen if there were no friction in the pivot (which I think is more important than air resistance- apparently your teacher oils his pendulums better than I do!) and plenty of gravity. The pendulum would move back and forth forever without coming to rest. Consider, on the other hand, a pendulum with no gravity but friction and air resistance. The pendulum would not, even from the start, swing back and forth but go "round and round". Eventually, however, friction and air resistance would "steal" all of the energy and the pendulum would come to a stop.

I said, above, that gravity has everything to do with where the pendulum comes to rest. Obviously, with gravity, the pendulum will come to rest at its lowest point. Without gravity the pendulum will come to rest where ever it happened to be when its energy ran out.

4. Oct 18, 2006

### mathman

Gravity is one of the forces that makes a pendulum go - the other is the initial kick to get it started. If there was no friction (or air resistance) it would oscillate forever.

Specifically, gravity pulls the pendulem to the bottom of its arc, where its velocity is enough to keep it going to the top of its arc, where it comes to a stop and turns back down. The arc length will be constant in the absense of friction or air resistance.

5. Oct 18, 2006

### onestarburns

thanks

once again you guys have helped a great deal. thanks alot, and i must say it is great to have such an awsome sorce of information from such a wonderful community.
thanks: james