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Re: [Phys-l] two questions



Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
A few of my science colleagues were discussing some physics/chemistry
concepts.

1. Does anyone know how a Mexican jumping bean works? I believe there is
an insect larva inside that can jump. How, then, does the bean rise upward
since the forces are internal (and thus momentum is conserved)?

Even if they /don't/ actually jump (I've never seen one), there's nothing
a prioiri impossible about them jumping. Imagine JD's hamster ball, but
make it (WLOG) a box, for ease of visualization. Start with the box's
bottom on the floor, and the hamster on the bottom of the box. The
hamster /can/ (if hamsters could) jump ferociously upward; the normal
force of the box's bottom (which is related to the floor's N on the box)
can accelerate the hamster up just as if the box weren't there. Until the
hamster hits the top of the box, nothing out of the ordinary happens - the
ham's in freefall, etc. When the hamster hits the top of the box, there's
a collision, which is completely inelastic for a brief time (hams being
squishy, and all). During this, the ham and box are co-moving, and the
box (ball(bean)) comes off of the table. Shortly, the ham and box will
uncouple one way or another, dictated by the details of the collision, but
both will end up back on the ground.

So: conservation of momentum? Sure. The ham (and the ham-box system)
gains momentum when it is acted upon by the large N from the box's bottom
(from the table, in the system-view) (An external force). As the ham
moves upward in the box, the /system's/ COM moves upwards, too. The
interaction of the box and ham comes while the COM is still moving upward,
but while it's accelerating downward (due to gravity, another external
force). A short time later, the COM moves back down to where it was
before. The COM's y vs. t graph is a parabola, though the motion of the
components (ham, box) of the system have funny-looking motions.

Josh Gates
Stoneleigh-Burnham School
Greenfield MA


2) Water is a polar molecule, and a charged balloon will easily bend a
stream of water. Hexane is not a polar molecule, and chemistry demo books
say the stream should NOT bend. But if you put hexane in a buret, a
charged balloon DOES bend the stream. A charged balloon should attract any
neutral object, right? Something seems wrong here...

Can anyone assist me with these questions?

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l