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Re: Frames of Reference



Does the physicist, checking for momentum conservation in a bubble chamber
photograph, worry about the earth providing a moving frame of reference
or do they operate as we do in a conventional lab, assume the lab is an
inertial frame?

He doesn't have to. The quantity which is important in collisions is the
impulse exchanged. These interactions take place over such a short time
that any impulse due to gravitational forces is utterly unobservable,
though the gravitational forces are in principle quite observable.

That's a good problem to give your students, however. Many will say "The
gravitational force is just too small to matter". That is not the case, of
course. If the particles stuck around for a tenth of a second in the spark
chamber they would fall just as far as a tennis ball in the same time. The
question will test whether or not they are thinking.

Leigh

Howdy,

I agree with you completely on this Leigh. I always derive P-Conservation
from the Impulse-Momentum Theorem and looking at multiple objects over the
same time period.

By the way, to kill a dead horse, I call weight the gravitational force one
body applies on another: the gravitational force the earth applies on a
body near its surface is the body's weight on the earth. The "apparent
weight" (i.e., the weight you feel-yes I do use that term!!!) is more
closely related to forces your surroundings apply on you. Oh well, there
is more than one way to skin a cat.

Good Luck,
Herb Schulz
(herbs@interaccess.com)

P.S. Where do all those terrible references to doing horrible things to
animals come from?