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[Phys-l] Teaching torque




I think it is EXTREMELY important that students know what they are "torquing
about".

(I guess this applies to everyone, actually.)

I like the subscript idea for keeping track of this.

When summing torques and setting equal to zero in a rotational equilibrium
problem I always have my students document not only the point about which all
the torques are all summed, but the convention for positive and negative
torques in that equation. (We deal with only 2D problems in College Phys, so
CW and CCW suffice)

SL

Date: Sun, 27 Aug 2006 20:07:12 -0700
From: Leigh Palmer <palmer@sfu.ca>
Subject: [Phys-l] Teaching torque


I consider torque to be a somewhat difficult topic for students. One
of the most frequently asked questions from first year students that
I answered in my office was "What is torque?" My first sentence in
answer was always "Grab the end of this meter stick and hold it
tightly." I then twisted the meter stick. "That's torque", I would
then say. Torque is twisting force, sort of. Then, of course, I got
more formal. In first year we lie to students about torque. We almost
never speak about torques as being referred to a point; we always
speak of a torque with respect to an axis. We should always be
prepared to recognize this when we teach mechanics again to the
continuing students. A "torque with respect to an axis" is merely the
component parallel to that axis of a torque with respect to a point
on that axis.

I always asked first year students to write the symbol for torque
with a subscript representing the axis to remind them that torques
must usually be referred to some point or axis. It is a small point,
but I think it helped straighten thinking about torques. Some
teachers think that subscripts should be avoided in elementary
courses. I think subscripts serve a useful purpose in many cases.

Leigh

--
Stuart Leinoff
Professor of Physics
Adirondack Community College
(518) 743-2256






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