Thread: rod in freefall
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Doc Al
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Nov22-12, 03:38 AM
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Quote Quote by aaaa202 View Post
This has been brought up numerous times but I don't really understand it. Consider a rod in freefall.
If you put your coordinate frame in the center of mass of the rod, there will be no torque around it and the rod as a whole will follow a straightline down. But now put a coordinate frame on one of the end points. Apart from the gravity pulling down on the rod as a whole, there will now be a net torque on the rod (because gravity acts in the center of mass).
What goes wrong with this picture, because clearly the rod doesn't rotate!
The problem is that you are you using an accelerating point as your 'pivot'. Torque about such an accelerating point does not simply equal the rate of change of angular momentum, unless that point happens to be the center of mass.

See my post in this thread: http://www.physicsforums.com/showthread.php?p=4097976