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On 10/20/2006 01:12 PM, David Abineri wrote:
The reason I asked the question is that, after many years of teaching Physics, that the idea of separating the concepts of weight and mass seems to be decidedly non trivial especially to thinkers who are trapped on the earth's surface. We seem to take it for granted today, especially in the space age,
I agree that the distinction is nontrivial ... but nontrivial
is not the same as incomprehensible.
Even before the space age -- going back to 1638 if not before --
the distinction could be perceived using pendulums, or using
stuff rolling on horizontal planes or gently-sloping planes.
Galileo was all over this. He even had pulleys set up so that
a small mass moving vertically could balance a larger mass moving
on an inclined plane.
Also, as I've said before, for pedagogical purposes one should
focus on the best available evidence, not the most ancient
evidence. In this case the oldies are also goodies (pendulums,
horizontal planes, gently-sloping planes, and pulleys). In
addition to good old rolling, we have modern ultra-low-friction
To teach about mass, my favorite activity is pushing a car. Not a
toy car, but a full-size honest-to-goodness car. Take a "field trip"
to the parking lot. On a level surface, weight is irrelevant, in
the sense that nobody is going to /lift/ the car. On the other
hand, they can push it sideways. It "resists" pushing in proportion
to its mass. A megagram is enough mass to be interesting.
Friction is small enough to be a non-troublesome correction term.
Once the car is moving, it "resists" stopping ... again in proportion
to its mass. Safety suggestion: Use a rope, so the kids can stop
the car by /pulling/ on the rope, so that nobody needs to get in
front of a moving car. Put a driver in the car, to apply the brakes
in case of emergency.
If you want to teach them something with real-world practical value,
teach them to push with their legs.
You can get all fancy and measure the distance versus time with
various numbers & sizes of pushers ... but you don't really have
to. Just the experience of pushing gives folks a priceless "feel"
for what mass is.
Little kids can have even more fun with a multi-megagram railroad
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