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Re: [Phys-l] What is energy? (a little long)



I have no criticism of Cliff's approach ... except that it is,
as noted, a little long. It is somewhat longer and more indirect
than it needs to be IMHO.

Starting from the idea of mass as "amount of stuff", I would
start by emphasizing horizontal motion in flatland. No vertical
motion is allowed, not even "virtual" motion (as in the principle
of virtual work). Weight is meaningless if things are constrained
to move horizontally. OTOH mass is still meaningful, and inertia
is still meaningful. This reminds me of one of the famous quotes
from my mother:
"Hey, where are you going with that railroad car?"

For the next level of detail on that -- i.e. a direct, hands-on
demonstration of inertial mass -- see
http://www.av8n.com/physics/mass.htm#sec-car-push

As another way to approach and/or reinforce this point, I would
show a short movie of life in the space station, then challenge
them to design a stuffometer (i.e. mass measuring instrument)
that works in weightless conditions. That requires them to focus
their attention on the idea of inertial mass.

In contrast, in any case where there is a nonzero g field,
there is a huge temptation to fall back on a measurement of
weight ... which is perfectly practical and reasonable, but
fails to illustrate the point we're trying to make.