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Re: Work-energy worries




I have to return to something like my previous example. I stretch and
then release a mass on a horizontal spring (on the surface of a
heavy, frictionless table with the other end of the spring attached
to a post on the table). System A = mass. System B = spring + table.
System B does work W_A on the mass as the spring relaxes.
Consequently the mass gains KE.

Now for the key question: How much work W_B does the mass do on
system B? Justify your answer.

W_B = -W_A -- Just because you say the table is "rigid" doesn't mean the
table doesn't move -- See below...

Neglect the friction between the skates and the ice and assume that a
person of mass m pushes off a rigid, stationary wall starting from
rest by extending his arms a distance d,

John, I see this question as a fallacy in your pseudowork ideas. There is
no thing as a "rigid stationary wall" -- If one pushes on a wall which is
truly rigidly fixed to the Earth, at the very least, the Earth moves.


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

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.