🚀 go-pugleaf

On Wed, 10 May 1995, Mervin Koehlinger wrote:

> The football tackle involves another force which is not mentioned in the
> original problem.  That force is gravity, which is responsible for the
> rotation.  It would also be difficult to consider a human body as a rigid
> object.

and, Nancy Douglas wrote:

> Maybe it's late in the semester and I'm tired and sleepy.  BUT when you
> jumped in the air playing football and were tackled didn't your body
> rotate because of the effect of gravity on your center of mass?  Wasn't
> the orginal question about a force applied to the end of a rod occuring
> in space?

I don't understand this 'because of gravity' acting 'on your center of
mass' then 'your body rotates'. Nor do I understand 'gravity being
responsible for the rotation'. If I'm sitting here typing, the force of
gravity acts on my center of mass and I'm certainly not rotating. If I
jump in the air, I do not necessarily rotate, unless a torque were
applied as I jumped, or after I jumped, as if I were tackled. Therefore
wouldn't it be more meaningful to implicate that other force, the one
producing a torgue, rather than gravity? You seem to be glossing over
important details here, namely mathematical details.

If a meter stick balanced on its end topples, it gains angular momentum.
What is the cause? Gravity? No. The reaction force of the table or floor
on the lower end of the stick? No. Both of them *together* constitue the
couple whose torque causes the *change* in angular momentum.

These flippant 'because' answers are never adquate to understand physics.
I urge the adoption of my ground rule: Any answer with the word 'because'
in it loses x points no matter how good the rest of the answer.

        -- Donald