Meteorologist Ray Petelin is back with another home science lesson!

PITTSBURGH (KDKA) — We have discussed gravity in a couple different experiments. This time, I am going to show you something that may seem almost impossible to do. Then we will discuss how it works, then I am going to show you how to do it. It all has to do with the “center of gravity”.

This is a 10-dollar bill. Any bill will do, but what is special about this one is that I balanced a penny on its edge. If you have ever tried to do this, it may seem impossible, and the reason most people can’t get a penny to balance is that they cannot find the penny’s exact center of gravity.

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The center of gravity is defined as a point from which the weight of a body or system may be considered to act, so it is where the total weight of something is thought to be concentrated. That means for this trick to work, the penny’s center of gravity needs to be directly over the very thin edge of the bill. If you are off by even a little bit, one side will be heavier, and the penny will fall off.

Before showing you how to perform the trick, first let me show you how to find the center of gravity of something easier to balance. Then we will use what we know of that to balance a penny on the thin edge of a dollar, or any type of bill.

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For this, we need a yard stick and your pointer fingers. Rest the yardstick on your fingers. As you do this, gravity is pulling the yardstick to the ground and your fingers are providing an upward force.

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This is holding the yardstick up, as you would expect. Now, slide your fingers together. You will notice that your fingers come together at the exact middle of the yardstick. Since a yardstick is 36 inches, your fingers will come together at 18 inches, which happens to be the yard stick’s center of gravity.

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If you pay closer attention while sliding your fingers together, you will notice that at times one finger slides more than the other. That is because the finger with the least load slides, while the finger with more load from the yard stick will stay put at that point of the yard stick. That “back and forth” between which finger is holding more yard stick will happen until both fingers meet.

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If you add weight to one side, obviously that will throw the balance off, but if you perform the same steps, your fingers will meet at a different spot on the yardstick, but that different spot is still the center of gravity. It is where the whole thing balances!

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Since we can’t use our fingers to find the penny’s center of gravity, we will do it another way.

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If we fold the bill in half, two parts of the bill will support the penny, sort of like our fingers with the yard stick.

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As you stretch the bill out, that same “back and forth” will happen between which half of the bill is supporting the penny more will occur, just like the yard stick!

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Eventually, the penny is left with its center of gravity directly on the edge of the bill.

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