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Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo





To demonstrate equilibrium i.e. balance of forces, put
the puck "here" and "there" and observe that it remains
at rest. Then give it a push and observe uniform motion.

The vertical forces are balanced. The horizontal forces
are nearly zero.


Just out of interest, how do you justify the claim that the forces are balanced, without chasing your tail?


If you want nonzero but balanced horizontal forces, you
can have a puck inside an annular puck, with rubber bands
connecting the inner puck to the outer puck. If you
start the thing moving properly, it will move as a unit,
and the rubber bands will not show any extension relative
to where they were in the nonmoving state.

That's nice!


As a related but more advanced demo, I am fond of dropping
an accelerometer.
http://www.av8n.com/physics/accelerometer.htm
The accelerometer right-side-up registers +1 gee.
The accelerometer upside-down registers -1 gee.
The accelerometer held horizontally registers zero gee
(since it measures the projection of g along its axis).
The accelerometer in free fall registers zero gee.

This kills off the Newtonian idea that the laboratory frame
is an unaccelerated frame. Any decent accelerometer sitting
on the laboratory table will register +1 gee. This upholds
and illustrates Einstein's principle of equivalence.

And that one is especially nice. I imagine you'd generate some interesting discussions around the fact that you get a non-zero reading all the way around the globe too ;-)

As an aside, if the accelerometer is a 'mechanical' device, it really demonstrates a very restricted 'equivalence principle' that was in use well before Einstein (Newton used it to help solve problems concerning the kinematics of Jupiter and its moons, for example). If the demo was intended to be used to introduce general relativity, I think I'd prefer an accelerometer that relied heavily on electromagnetism, and/or other non-mechanical principles, for its operation.


Derek

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