This Week's Experiment - #254 An Easy Swing
I was looking at several different possibilities for this week's experiment
when I came across some of my old notes from an Amusement Park Physics
program that I taught at the Cumberland Science Museum in Nashville, TN
several years ago. This was a summer workshop where we spent two days in the
lab, followed by two days in an amusement park, performing experiments. One
of my favorite parts of this workshop was showing how science is used to make
the games harder than they look. This one is based on a carnival game where
you try to swing a weight on a string and knock over a bottle. To try this,
you will need:
two chairs
string
a broom handle or similar long rod
a small, heavy weight that can be tied to the string
a pen or pencil with one end flat enough that you can stand it on end
The first thing we need to do is to make a pendulum. To do this, place the
two chairs about 3 feet apart, with their backs towards each other. Place
the broom handle across the backs of the two chairs. Tie one end of the
string to the heavy weight. I used a fishing sinker, but you could use a
heavy key, several coins taped together, or some similar weight. Stand the
pen on its flat end, directly under the center of the broom handle. Tie the
other end of the string to the center of the broom handle, so that the weight
is just low enough so that it will hit the end of the pen.
Now we are ready for the game. It seems very simple at first. The idea is
to pull the weight and release it so that it will knock over the pen. Easy,
right? But wait. There is one more thing. It must not knock the pen over
as it swings away from you. Instead, you must swing the weight past the pen
and have it knock the pen over as it swings back to you. It still sounds
easy. Try it. Yes, it can be done. It takes practice.
Why is this so hard? Move the pen and swing the pendulum a few times. Just
watch it swing. If you swing it forwards, then it reaches a point where it
reverses direction and comes back to you, and it goes almost as high as it
did at the other end of its swing. It does not go quite as high because it
loses a little of its energy to friction. The same is true if you swing the
weight to the right. It moves to the right and then reverses to swing back
to the left. It will swing left almost as high as it did to the right. Can
you swing the weight to the right so that as it swings back, it will stop
when it reaches the bottom of the swing and not swing up to the left? No, as
long as it does not hit anything, it will continue to swing. Inertia will
keep the weight moving until gravity, friction of some other force slows it
down. That is the clue for why the game is so hard.
If you swing the weight so that it passes on the right side of the pen, you
have combined swinging the weight forward with swinging it to the right, to
make it miss the pen as it swings by. If you swing it forward and to the
right, then when it reaches the point where it stops and reverses its course
you would expect it to swing back and to the left. That is exactly what
happens. If it misses the pen on the right side on the first half of the
swing, it will miss the pen on its left side as it swings back. As long as
the pen is standing directly under the point where the weight is tied, this
is a very difficult task.
There are ways to win this game. One is to just barely miss the pen on the
outward swing. Most people try to swing the string far to the right,
thinking that it will make a small circle and hit the pen on the way back.
Really, the farther to the right you swing the weight, the bigger the circle
and the farther it will miss to the left as it swings back. If you can just
barely miss the pen on the outward swing, it will lose a little of its
energy, due to friction, and will hit the pen on its way back.
Another way is to give the weight a spin as you release it. This spin alters
the path of the swing and with practice you can learn just the right amount
of spin to get the weight to hit the pen. The people that run this game are not
cheating and the game is not rigged. They are just using science in a game
that looks easy but is not.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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