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Hanging a Hammer

This experiment involves balancing a hammer, a ruler, and some string. It is one of those balancing tricks that seem as though they just should not work, even though you understand the science behind them. I first learned it when I was eight years old, but it is still one of my favorites.

You will need:

• a hammer
• a ruler
• string
• a table or chair

Take a piece of string about 6 inches long, and tie it into a loop. Be sure the string is strong enough to hold the weight of the hammer. Hold the hammer's handle against the ruler, with the end of the handle at about the middle of the ruler. Put the loop of string around the hammer and the ruler and slide it about 2/3 of the way towards the head of the hammer. Now, carefully lift up on the free end of the ruler (the end near the head of the hammer). The other end of the ruler should press against the end of the hammer handle. The head of the hammer should extend beyond the end of the ruler.

Place the end of the ruler on the edge of the table, so that the head of the hammer is under the table. The entire thing should balance there on the edge of the table. You may have to adjust it a bit to make it stay. If it won't balance, try making the string loop shorter and try moving the hammer towards the table. The head of the hammer must extend beyond the end of the ruler.

Why does it balance? The heaviest part of the whole thing is the head of the hammer. Gravity is pulling the hammer down. Look at the hammer handle and the string. It looks a lot like a playground seesaw or teeter totter. When one end goes down, the other end goes up. Gravity pulling down on the hammer head pushes up on the end of the handle. The handle pushes up against the ruler.

The ruler and the string also act as a seesaw (a simple lever). The hammer handle pushes up on one end of the ruler, which causes the other end of the ruler to push down, against the table. The string is the balancing point, which is called the fulcrum. By combining the two levers (the hammer handle and the ruler), the more weight you add to the hammer head, the more firmly the ruler will balance on the table.

You can also experiment with moving the hammer so that more of it extends under the table. That will cause the ruler to tilt upwards, making it look like an even more impossible situation.