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Re: [Phys-l] Statics conundrum



Agreed, but there is still utility, in some situations, in writing the additional moment equations. If a horizontal plank is resting on two supports, then moments about one support give the value of the other support force and vice versa. Even though the torques are equivalent, there is value in writing them both anyway. We get away with this because there are really two equilibrium conditions in this case: translational and rotational.

But you can't combine this approach with an algebra error, which is what I did, and end up solving for 3 variables with two equilibrium conditions :-(

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Monday, March 15, 2010 2:11 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Statics conundrum

On 03/14/2010 07:32 PM, LaMontagne, Bob wrote:

Why are you restricting the solution to only one moment equation?

Maybe because there is a famous theorem that says when
all the forces are in balance, the torque around one pivot
is equal to the torque around any other pivot.

So writing additional moment equations is guaranteed to not
tell you anything you didn't already know.

==========

More generally, the torque around one pivot differs from
the torque around any other pivot by an amount given by
(how much you moved the pivot) /\ (net unbalanced force)

Remark: This is a very useful theorem.

Homework: prove the theorem.

Hint: It's a one-liner.

qrsvavgvba bs gbedhr + qvfgevohgvir ynj
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