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# Torsional Pendulum Query? watch

1. Hey all,

Ok, I have a question. Lets say you have a rod, and it's attached to a wire in the exact centre. Different amounts of weight can be added or removed at either end of the rod, without affecting the overall length.

Now, i'm fairly sure the pendulum will have a restoring force, say R, for a given number of turns, and that we can assume this wont change too much. So, for say 40 turns, the force would be constant.

Now, what I want to know is this; Would adding masses slow the rotation of the pendulum, as the same R value would have to move more mass? I'm fairly certain, but not entirely sure.

_Kar.
2. You need to be careful with the terms here.
The pendulum wire will apply a torque T to the rod, and this is what causes the rotation.
The angular acceleration of the rod is given by

where alpha is its angular acceleration and I is its moment of inertia about its axis.
The moment of inertia will increase if you add weights to the end.
If I increases for the same T then alpha will decrease.
So you are correct in your assumption.
3. (Original post by Stonebridge)
You need to be careful with the terms here.
The pendulum wire will apply a torque T to the rod, and this is what causes the rotation.
The angular acceleration of the rod is given by

where alpha is its angular acceleration and I is its moment of inertia about its axis.
The moment of inertia will increase if you add weights to the end.
If I increases for the same T then alpha will decrease.
So you are correct in your assumption.
Ah, perfect.. We don't actually study torsion, nor torsional pendulums, but I was curious. Thanks a lot

_Kar.

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