🚀 go-pugleaf

Guntbert Reiter writes:

>Imagine the impact resulted in pure
>movement without rotation. More exactly spoken: the result would
>be a linear acceleration of the CM. By Newton's laws (inertia...)
>we know that from this acceleration the CM should "feel" (I just
>can't remember the correct term) a corresponding force pointing
>in the opposite direction. So these two forces build a couple and
>produce a torque.

Guntbert,

Imagine all you want, but you've got a powerful adversary in the
form of Newton's Laws.  (Do some of us really imagine that one
could tap on the end of a freely mobile, originally stationary
pencil in a direction perpendicular to its length and find that
the result of the tap is pure translation?)

You are apparently appealing to the "inertial force" that arises
in the (briefly) accelerating frame of the object's CM. I happen
to be a fan of inertial forces and you are right that the couple
produced by the impact and the equal and opposite inertial force
explains the rotation observed in the accelerating frame.  (It
also explains the LACK of acceleration observed in the
acceleratING frame.)  But you STILL need to account for the
rotation that is observed in the inertial frame and there IS only
one force in that frame.  Fortunately, and not surprisingly,
Newton's laws predict this result.

John

John Mallinckrodt
Cal Poly Pomona
ajmallinckro@csupomona.edu