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*From*: John Mallinckrodt <ajm@CSUPOMONA.EDU>*Date*: Thu, 17 Mar 2005 09:44:08 -0800

Of course the rotational form of the second law to which you refer can

always be used with an accelerating origin provided one accounts for

torques due to so-called "frame forces"*. In those special choices of

origin for which frame forces produce zero net torque one may apply the

law thoughtlessly without suffering an error in the answer. However,

would it not be better to apply the law thoughtfully and recognize that

the frame force forces contribute zero net torque with proper choice of

origin? Is this so different from choosing any other origin because it

simplifies calculation?

Leigh's good words of caution wrt the effects of frame forces remind

me of one of my favorite reference articles on this topic:

Fredy R. Zypman, "Moments to remember---The conditions for equating

torque and rate of change of angular momentum," Am. J. Phys. 58, 41

(1990).

In fact the abstract alone is a nice reference for those of us who

choose to almost but not quite thoughtlessly apply the law:

"It is a well-known but frequently forgotten fact that the net

external torque on a system of particles is equal to the rate of

change of angular momentum only if the point about which moments are

taken satisfies certain stringent conditions: The point must have

zero acceleration, or it must be the center of mass, or its

instantaneous acceleration must be directly toward or away from the

center of mass. ..."

John Mallinckrodt

Cal Poly Pomona

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**References**:**[Phys-L] Re: conservation of angular momentum question***From:*Leigh Palmer <palmer@SFU.CA>

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