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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.
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.
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.
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