This week's experiment is a result of an e-mail from Karen H. She asked about past experiments dealing with friction. As I began looking, I discovered that I had been ignoring this interesting subject. This week I will correct that. You will need:
- a rubber band
- a shoe
Friction is a subject that I thought I knew a lot about, until I began to really look at it. Then I realized that the subject was a lot more complex than just observing how things rub together.
Tribologists (scientists that study friction) divide friction into static and kinetic. To see the difference, cut a rubberband, so that you have a long string of rubber. Tie one end to a shoe. Set the shoe on the floor. Grasp the other end of the rubberband and begin pulling. Pull gently at first and then pull harder and harder until the shoe begins to move. Use the ruler to measure how long the rubberband stretches before the shoe begins to move. Before it begins to move, it is experiencing static friction between the shoe and the floor.
Once the shoe starts moving, measure how far the rubberband is stretched as you keep it moving. You will probably notice that it is not stretched as far as when you were trying to get it started. When it was sitting still, you had to overcome the static friction (as well as inertia) to get it going. Once it was going, you had to contend with kinetic friction, which is less that static friction, if everything else stays the same. In other words, once you get the thing moving, there is less friction to resist its movement.
How can we change the amount of friction? You could try using a different surface. Spread some aluminum foil on the floor and place the shoe on top. Try pulling the rubberband again and measure the static and kinetic friction. Did the shoe slide across the foil, or did the foil slide across the floor with the shoe? One way, you are measuring the friction between shoe and foil. The other you are measuring the friction between foil and floor. Try it both ways and compare.
Add some weight by putting some rocks into the shoe. Try again and see how increased weight changes the friction.
Try adding some roughness by pouring some salt or sugar under the shoe, or placing it on a sheet of sand paper.
Tape the foil to the floor and rub it with a little cooking oil. Then try the shoe again, measuring static and kinetic friction. What did you notice? Does this give you an idea of why they use oil as a lubricant in cars and other motors? Can you think of why the layer of oil would change the friction?
Kind of material, weight, roughness and lubricants can all have an impact on the amount of friction. Of course, tribologists argue about these all the time. Some say that roughness is different from friction. Others say that friction is a result of chemical attractions. Some even say that friction is due to conversion of energy into sound waves, which explains why dragging something heavy always seems to make such a terrible screech. It is interesting that even something as common as friction can still cause professors to argue over how it works. It is nice to know that there are still so many mysteries in science, and anything that can make teachers argue among themselves is a good thing.