[Phys-L] Re: conservation of angular momentum question

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

> 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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