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I have read two very similar yet conflicting threads on this site involving a pure torque applied to one end of a floating body in free space (no air resistance).

Torque applied off the center of mass - This answer says if a pure torque is applied to one end offset from the center of mass, the body will simply rotate around the center of mass without the motion of the COM changing.

However,

Reaction of applying a torque on a rigid body in space - In this question I asked earlier, the answer to nearly the same question is that the motion of the COM will change due to a pure torque.

So, which one is it? (I suspect my confusion might be an improper definition of a pure torque?) Note: Assume the rod is initially at rest in space and the frame of reference is a stationary observer.

Jonathan Lemon
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1 Answers1

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The second question there (yours) isn't about a situation where there is "pure torque".

You only have one force acting on the object, so it creates a torque and moves the centre of mass because there is a net force. A "pure torque" requires a "force couple" where the two forces cancel out on the centre of mass as $0$ net force; but due to the direction of the forces, it still produces a net torque on the object, which causes rotation without translation (A.K.A. pure torque, pure moment, or pure rotation).

Another interesting thing about a pure torque is that the moment is independent on the point you are measuring torque about; which is a special feature of pure torque. If you apply a pure torque, the moment vector at one end would be the same as the moment vector of the centre of mass, which is an interesting result not typical of non-pure moments.

JMac
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