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Re: Energy, etc. (third law/forces/work)



I have to agree with Jim on this one. I used to have a question on my final
exam "If one morning all the people in China start jogging towards the East
(and keep on jogging), what happens to the length of a day? While the
question is really trying to trigger a conservation of angular momentum
thought, the process does demand that there be a force on the earth
(actually a torque) so as to change it's rotational speed. That force is
the third law pair to the force that the earth provided on the billion plus
runners to accelerate them to jogging speed. To be sure, after the initial
step each ensuing step has a stopping and then restarting component which
tend to cancel, but until all the joggers stop there has been a net torque
on the earth. Of course even with a billion runners the effect is small,
but who hasn't told a class that they were going to move the earth--and then
dropped something?

Rick
-----Original Message-----
From: Jim Green <JMGreen@SISNA.COM>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Friday, August 06, 1999 12:43 PM
Subject: Re: Energy, etc


walking along a level path. We could look at walking at a steady pace
but let's take the situation when the person starts walking from rest.

Now the Earth rotates.

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen