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Re: [Phys-l] Equations




----- Original Message ----- From: "John Denker" <jsd@av8n.com>


Dan Crowe wrote:
"Consider a turntable, rotating uniformly. A puck sits on the
turntable. I can measure the mass (m) of the puck, and I can measure
the force (F) required to hold the puck in position relative to the
turntable ... but it would be perverse to suggest that the acceleration
(a) is _caused_ by F/m."

It is not perverse, and I will continue to state that the force on the
puck causes it to accelerate.

That depends on what "it" refers to.

What I _meant_ to say is that the puck sits at a point "A" on the
turntable. We can calculate the acceleration of point "A" by means
of measurements on the puck, but it would be perverse to say that
the acceleration of point "A" is caused by F/m. In particular,
point "A" would continue to accelerate just the same in the absence
of the puck.
_______________________________________________

But there is a net force (F') acting on mass (m') of the turntable material at point "A" that cause it to accelerate. We've just been clever to use the motion of the puck and the nature of the centripetal acceleration to provide a numerical value for the acceleration, but a = F/m must also be F'/m'.

Rick

********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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