Jan. 14, 2004

Your Center of Gravity

To do this experiment you will need:

• wooden block or blackboard eraser
• weights
• handkerchiefs

Where do you think your center of gravity is located? To find out, lean forward and balance your body on the back of a couch. At what point do you balance?

You can see how important it is to keep your center of gravity above your feet by doing the following experiment. Stand with the entire right side of your body, including the side of your right foot, in contact with a wall. Now try to lift your left foot. What happens? Does your body want to rotate about your center of gravity?

Now stand with your back against a wall. Be sure your heels are touching the wall. Drop a handkerchief just in front of your toes. Try to pick up the handkerchief without bending your knees or moving your feet. What happens when your center of gravity gets beyond your feet?

Do you think people's center of gravity will be different for boys than girls? Here is an experiment to find out.

Get down on your hands and knees and measure one cubit straight out from your knees, as shown below. (A cubit is the length of your forearm from the elbow to the tip of your middle finger.) Place a wooden block or a blackboard eraser on its edge one cubit from your knees. With your hands behind your back, lean forward and try to knock the block over with your nose. Can you do it, or do you fall on your nose instead? Whether you can do it or not has nothing to do with how clever you are; it depends only on how you are built.

Have a number of different people try this experiment. Are girls generally more successful than boys?

As you lean forward, your center of gravity also moves forward. What will happen if your center of gravity gets beyond your knees?

Repeat the experiment with some weights in your back pockets or tied to your calves with handkerchiefs. How will these weights change your center of gravity? Do they make it easier for you to tip the block over?

Do you see now why it is so hard to walk a tightrope or stand up in a small boat? Why is it dangerous to pull a chair away from someone who is in the process of sitting down?

Reprinted with permission from Science Project Ideas in the House, Rev. Ed. by Robert Gardner. © 2002 by Enslow Publishers (www.enslow.com).

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