Re: Newton's 3rd law? was Re: inertial forces (definition)

[paul o johnson <pojhome@FLASH.NET>]

Leigh Palmer wrote:

> I think that after seeing so many free body diagrams with gratuitous "centripetal
> forces" included for no
> conceivable reason other than the fact that a physics teacher or a textbook told
> the student it must be there, I've decided to try to do things differently.

Perhaps I'm naive, but I have a hard time believing that physics teachers today do
that. Students should be taught to use free object diagrams in both equilibrium and
non-equilibrium conditions. Far from being gratuitous, the centripetal force (that
is shown in the diagram ONLY when the object is experiencing an acceleration
perpendicular to its velocity) is necessary to justify and explain the non-uniform
motion. The student should be taught that the force MUST be there because of the
motion; the task for the student, then, is to discover the source of this force.

poj