conservation versus friction

[JohnDenker]

In section 11.5 on page 272 it says

if it weren't for friction

It seems to me that friction is irrelevant to this discussion of whether angular momentum is constant, or conserved, or whatever.

• Angular momentum is conserved, whether friction is present or not.
• Angular momentum can be transferred by frictional or non-frictional forces.

It would be much better to say:

Indeed, in the absence of external forces, objects in circular motion would continue their circular motion ...

As a partially-related point: In the absence of external forces, they do not merely "tend to" continue. They unconditionally continue.

[ericmazur]

Small quip: I'm not saying that angular momentum is not conserved if friction is present! Agree on the "tend"

[JohnDenker]

Gaaack, my suggested wording is not quite correct. It would be better to say either:

Indeed, in the absence of external forces, a rigid object in circular motion will continue its circular motion ...

or:

Indeed, in the absence of external forces, objects that have angular momentum will maintain constant angular momentum forever ...

Rationale: Unlike linear momentum, it is possible to "hide" angular momentum internally. Satellites in orbit routinely use internal gyros to change the rate of rotation of the overall object. Gymnasts use the same idea. Angular momentum is conserved, but angular "motion" is not quite the same thing. Mass cannot be changed, but moment of inertia can.