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Re: ENERGY WITH Q



At 10:46 AM 10/30/01 -0600, RAUBER, JOEL wrote:

Would it help (or is it true) to say:

But is it true that F_ext dot ds_cm + torque_ext times dtheta is Delta KE?
Where the external torque is measured relative to the cm.

1) It's not true. Consider a device with a "front wheel" and a "back
wheel". Starting from rest, I spin up the wheels, counter-rotating. The
net torque around the CM is zero. The motion of the CM is zero. The
conjectured expression above is zero squared. Yet the kinetic energy
increases.

2) You can construct a true statement by picking the object apart into
pieces so small that they have no internal structure, and applying the
Work-KE theorem to each piece separately.

3) Useful? Item (2) is useful in certain special circumstances. Feynman
uses it to derive some facts about ideal gasses. But it's not very useful
in general. I can't imagine using it to analyze books sliding across
tables under the influence of friction.