This simple demonstration shows how lifting an object can increase
the height of its swing.
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1. Tie
a few feet of string to the handle of a plastic coffee mug (or any other
unbreakable object onto which you can tie a string). Tie the string good
and tight so the mug doesn’t fly off when you swing it.
2. Holding the free end of the string in your left hand, enclose the string
in a loose fist
with your right hand, leaving enough space at the center of your fist
for the string to slide freely. The mug should now be dangling below your
right hand.
3. With a small push or kick, start the mug swinging slightly.
4. Keeping your right hand perfectly still, pull the string sideways with
your left hand to lift the mug just after it passes through the bottom
of its arc. Then, when the mug is at the top of its swing, let the mug
drop back down by releasing some of the string.
5. If you continue to lift and drop the mug this way, you’ll find
you can get it to swing higher and higher. This method of adding energy
is the same one used by skaters moving on the curved surface of a half-pipe.
