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I quote: "Imagine standing still on a frictionless surface and
then pushing off a wall to get yourself moving. Since the points
of application of either the force on you or the force on the wall
do not move, no work is done either on or by you. However you gain
kinetic energy as you slide away from the wall."
Some people came to the rescue of those who do not understand
this mistery, by inventing something called "pseudowork", thus adding
to the general confusion. No "pseudowork" is needed !
Replace the human body by a brick. Replace the hand and the
pushing arm by a compressed spring. Then let the spring expand.
If you want to understand the motion of *the brick*, you
must pay atention to all forces acting on *the brick* (You may
forget the force applied by the wall *on the spring*) It is the work
done by the force applied by *the spring on the brick*, that equals the
change of kinetic energy of the *brick*.
Now replace *spring* by the arms and muscles and replace *brick*
by the rest of your body. The top of the arms push on the rest
of the body and if you multiply this push while the arms stretch
by and appropiate (non zero) displacement, you will get a non
zero increase in kinetic energy.
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This discourse fails to explain how the arm (spring) acquires its kinetic
energy, and since the arm is part of the person, an explanation as to how
it acquires its kinetic energy is needed for a satisfactory analysis of
the situation.
As a term, pseudowork has not be well received. Perhaps it is best that
it not be used when talking with students and not appear in introductory
textbooks. However, as a concept pseudowork is very appealing. In the
case in which a person pushes off from a stationary wall it allows one to
consider the motion of the entire person, and not the person sans arm (the
brick).
Gene
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Eugene (Gene) P. Mosca Phone 410-293-6659 (Fax 3729)
Physics Department 410-267-0144 Home
572 Holloway Road 610-683-3597 Summer
U.S.Naval Academy Mailstop 9C, Michelson Rm 339
Annapolis MD 21402 email: mosca@nadn.navy.mil
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