## Hands On Physics

### Extension 2: A Look at Friction

It turns out that you cannot build a good mathematical model without friction, but what, exactly, is it? The issue of friction was introduced into the model without too much discussion, so here is your chance.

Friction is a force that opposes motion. Push a car one direction and friction pushes the opposite. Reverse the direction of the car and friction reverses, too.

Friction always reduces energy. Without it, the bungee jumper would return to the same height it started. Does it? How high does it bounce back? Call the loss of height dh. The energy loss due to friction is:

Energy loss = m*g*dh

How much energy did you loose? Where does this energy go?

There are two kinds of friction in this experiment:

• Air friction. When you put your hand out of a speeding car you feel air friction. If when you feel the wind, you are actually feeling the friction of the passing air.
• Friction in the bungee cord. The bungee cord has internal friction. When you release a stretched cord, it warms and this can result in energy loss. Do you see why?

Which source of friction is most important? One way is to try to change one kind of friction without changing anything else. Air friction can be increased by making the bungee go faster or making it larger. The bungee friction can be reduced by substituting a spring.

Decide on your experiment and how you will explore friction and make the required measurements. Before you begin experimenting, tell your instructor what you plan.

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