Re: [Phys-l] Equations

["Dan Crowe" <dcrowe@sotc.org>]

In the equation a = F/m, all of the variables refer to the same system;
therefore, I assumed that John was discussing the acceleration of the
puck.  I agree that it is perverse to suggest that the force on the puck
causes the acceleration of the turntable.  But the force on the puck
does cause the acceleration of the puck.

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John
Denker
Sent: Thursday, April 27, 2006 11:08 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Equations

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.