For an object to balance, you have to have the center of gravity directly above the base. Well, reverse that to see that you can also balance an object by having the center of gravity directly below its base.
To try that, you will need:
- 2 forks
- a piece of apple, potato or other firm vegetable about two inches square
- a wooden toothpick or match stick
- masking tape
- a marker or ink pen
Start by sticking a fork into one side of the piece of apple. Then stick the other fork into the other side. You want them both angled downward slightly, to form a large "V" shape, with the apple at the point of the V, and the forks forming the two arms.
Stick the toothpick into the apple, in between the two forks, pointing in the same direction as the handles of the forks. Now you have a "W", with the toothpick forming the center point.
Place the point of the toothpick on your finger, and try to balance the forks. It works! The whole thing will balance quite easily. How?
There is nothing over the base, but then there is nothing below the base either. This is a case where an object's center of gravity is outside the object.
We can find that center of gravity with some masking tape and a pen. Balance the forks on your finger again. You want to stick a strip of masking tape from one fork to the other, so that it passes directly under the point where the toothpick is balanced on your finger. Then make a mark on the tape, directly under that balance point. That mark shows you the center of gravity for the object.
Now push the toothpick into the apple until only about half an inch sticks out. Again, balance it on your finger. The angle will be very different, but you will find that the mark you made on the tape is still directly under the balance point.
Remove the toothpick and stick it into the apple in a different spot. Try balancing it again. If you can get it to balance, you will find that the mark is again underneath the balance point.