Riding on the top bar of the swingset is a carriage, not
too different from a primitive skateboard, shown in black.
There was only one rope, looped around the five pulleys as
shown. In the diagram, the rope changes from blue to purple
at one point, but that is just artistic license, to make the
diagram easier to understand.
The purpose, of course, was so that we could fly around in
two dimensions.
This apparatus had problems with friction, and other problems,
but it worked after a fashion, especially if your brother
helped by manipulating the ropes shown near the right side
of the diagram.
1) Consider the sub-case where the objective is to model
a moon walk, i.e. 1/6th of normal gravity. Calculate the
appropriate mass for the counterweight (shown as CW in
the diagram).
2) Calculate the sideways force on the carriage. Assume
no helper. For simplicity, you may neglect friction.
3) After you understand what this apparatus does, look up US
patent # 3476385. In what ways, if any, is the professional
version superior to the little-kid version?
As you may have noticed, I am attracted to the physics of
the real world.
(Not everyone is. One of my colleagues who is much more
interested in academic abstractions said "John likes to
play in traffic." That's not entirely an apt description,
but it was good for laughs all around.)
There is a ton of interesting physics just lying around
waiting to be looked at. Generally it is slightly more
complicated than the non-nutritive sweeteners you find as
end-of-chapter problems ... but only slightly more. The
block upon fall and the little-kid version of the peter
pan apparatus have got more than the minimal number of
moving parts, but they yield to PVW. They require more
than one step, but not much more than that.
In some sense it is a test of character. As the hitch
hiker's guide says, "don't panic". If you see a problem
that looks a teensy bit complicated, just do the physics
and you'll be fine. These two example problems don't
require anywhere near the depth of thought that little
kids routinely apply to chess games.