Date: Thu Dec 20 11:22:13 2007

Author: Paul Nord

Subject: Re: two questions

Post:
Mexican Jumping Bean:
>> How, then, does the bean rise upward since the forces are internal
>> (and
>> thus momentum is conserved)?

That statement is actually incorrect.

Quick counter-example: Stand inside a box and make the box lift off
the ground. You can do it very easily.

More interesting counter-example: Juggling. Ask the following
question, "What force do the jugglers feet exert on the floor while
periodically throwing balls up in the air and catching them. You will
note that throwing the ball accelerates is upward. Catching the ball
also accelerates the ball upward (or decelerates it downward if you
like). The time-average force on the floor is a net increase.

Even more interesting: Setup a ballistocardiogram. Have a person lay
on a board that is sitting on some thin round metal rods. Affix a
mirror to one of the rods. Shine a laser on the mirror so that it
deflects to the ceiling or some distant wall. You'll see that the
person's body moves up and down in response to the beating of his or
her heart.

Paul


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