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Balance And Stability

You Need: 1 metre stick or a 1 m piece of wood dowel, 1 overhead projector pen (washable ink)

What to Do
1. Locate the centre of gravity of a metre stick (or any long, uniform piece of wood such as a length of dowel). The centre of gravity is the single point at or near the middle of the stick where you can support the stick horizontally without it falling or turning. Hold the metre stick on your two index fingers. Slowly slide your fingers toward each other. When they meet, they will have the centre of gravity ‘surrounded’. Try this several times to confirm that it works every time!
2. Mark the location of the centre of gravity with a spot of ink from a washable overhead pen.
Stable or Unstable? Notice that when the act of tilting an object raises the centre of gravity, the object will fall back to its original position when you let go. This object is stable. When the act of tilting lowers the centre of gravity, the object
will continue to fall on its own, and not return to its original position. This object is unstable.

Think About It!
1. Is a stick more stable when it is flat on the ground, or when it is standing on its end? Why?

2. Why does a football lineman crouch when a play is about to start?

3. Lay the stick flat on your desk. Gradually, tilt one end upward
as in Figure 2, then let go of the stick. Does it return to its original position or tip over another way? When you lifted the stick, was the centre of gravity raised or lowered?

4. Hold the stick in a vertical position, as in Figure 3. Push the top of the stick slightly to one side. Is the centre of gravity raised or lowered when you do this?

5. Let the stick go! Does it return to its original position?
Cory Kuch shows what to do.

Stable or Unstable? Notice that when the act of tilting an object raises the centre of gravity, the object will fall back to its original position when you let go. This object is stable. When the act of tilting lowers the centre of gravity, the object
will continue to fall on its own, and not return to its original position. This object is unstable.

Think About It!

1. Is a stick more stable when it is flat on the ground, or when it is standing on its end? Why?

2. Why does a football lineman crouch when a play is about to start?

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