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Rod in freefallby aaaa202
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#1
Nov2212, 03:25 AM

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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! 


#2
Nov2212, 03:28 AM

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#3
Nov2212, 03:38 AM

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


#4
Nov2212, 03:39 AM

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Rod in freefall



#5
Nov2212, 06:42 AM

P: 4,212

http://en.wikipedia.org/wiki/Fictiti..._straight_line The inertial force cancels gravity at any point of the rod. Regardless if the origin is in the center or the end: There is no net force on any part of the rod in such a frame, and thus no torque. 


#6
Nov2212, 06:57 AM

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The extra terms (beyond the torque due to external forces) you get when you calculate dL/dt about an accelerating point are equivalent to introducing that inertial force. 


#7
Nov2212, 08:33 AM

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In the frame of one of the ends, the rod gains angular momentum  by falling linearly to the floor.
The torque is present, and required for a linear motion downwards in this frame. 


#8
Nov2212, 08:44 AM

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#9
Nov2312, 02:12 AM

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#10
Nov2312, 03:00 AM

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