Balancing a Meter Stick

This is a simple and easy science experiment, yet it still surprises people.

We are going to find the center of gravity of the meter stick. That is the point where it will balance on your finger. This is very easy to do if you understand balance and friction. Simply hold your hands out in front of you about two feet apart. Stick out your index fingers, as if you were pointing at someone and lay the meter stick across them. Now slowly move your hands together. When your fingers meet, the meter stick will be balanced on your fingers. Notice the point on the ruler where they meet. It is the center of the stick. Now is when the surprising part starts.

You will need:

  • a yard stick or meter stick
  • a few coins or other small weights

Ahh, you say. Each hand moved half way and just happened to wind up at the balance point. Lets test that idea by moving the balance point. Place several coins near one end of the meter stick. This will make that end heavier, shifting the center of gravity and the point where the stick will balance. Again, place the meter stick on your fingers and move them together. Did it wind up in the middle again? No. Instead, the side with the coins is shorter than the other side. Again, you have found the point where the two sides balance. You can keep adding and moving weights and it still works. In fact, if you move your hands slowly together, you can't NOT wind up at the balance point.

How does it work? When you start moving your hands together, if one end is heavier, it presses down harder on the finger on that side. The increased friction keeps the ruler from sliding across that finger. Instead, the ruler slips over the other finger, until enough of the ruler is on that side to make it heavier. Then its increased friction causes the other side to start slipping. This back and forth slipping insures that your fingers will wind up at the center of gravity, the balancing point. This is a very handy experiment, as it is about the only way I can be sure of balancing my checkbook.

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