We’ve done a couple of gravity experiments, but we haven’t talked about another ever present force (think of it as gravity’s little sister) — friction.

Friction is the resistance that one object encounters when it is rubbing against another object. Friction is what makes things slow down. You use friction when you are riding your bicycle and want to stop.

Without friction things would just keep moving unless they hit something else. Without friction, you would not be able to walk. Your feet would have nothing to push against, and they would just slide backward all the time as if you were doing the moon walk.

Friction is a very complicated interaction between pressure and the type of materials that are touching one another. Here’s an  activity to help you get the hang of what friction is all about:

What You Need:

• About 5 different shoes (theydo not need to be stinky)
• A board, or a tray, or a large book at least 15 inches long and no more than 2 feet long.
• A ruler
• Paper
• Pencil
• A partner

First we’ll do a practice run with one of your shoes (sorry couldn’t resist the pun!):

What You Do:

1. Put the board, or whatever you’re using, on the table.
2. Put the shoe on the board with the back of the shoe touching the back edge of the board.
3. Have a partner hold the ruler upright at the back of the board so that the 12 inches end is up and the 1 inch end is on the table.
4. Slowly lift the back of the board, leaving the front of the board on the table. (You’re making a ramp with the board.) Eventually the shoe will begin to slide.
5. Stop moving the board when the shoe slides and measure the height of the board. The greater the friction, the higher the end of the ramp has to be lifted.
6. Before you test any more shoes, analyze all of the shoes.
   • Are the bottoms made out of the same type of material?
   • Are they light or heavy?
   • How much of the bottom will actually be touching the board?
   • Is there anything else about them that you think might have an effect on how much friction they have?
   • Form a hypothesis that explains why you think each shoe will have more or less friction than the other shoes.
7. Using your newly formed hypothesis, look at the 5 shoes you chose and make a guess at which shoe will have the most friction.
   • On a scale from 1 to 5 (or however many shoes you’re using) rate the shoes you picked. (1 is lowest friction and 5 is highest level of friction.)
   • Write the guess next to a description of each shoe on a piece of paper.
   • Test all of the shoes.

What Did You Discover?

Did you have any surprises with which shoe had the most or least friction? Compare the shoe with the most friction to the shoe with the least friction. Do you notice anything?

Usually the shoe that has the most friction has more shoe surface touching the board than most of the other shoes. Often the shoe with the least friction has the least amount of surface touching the board. Since friction is all about two things rubbing together, the more surface that’s rubbing, the more friction you get.

A tire on you car should have treads, but some race car tires will be absolutely smooth with no treads at all. Why? The race car driver doesn’t have to worry about rain or wetness so he or she wants every single bit of the tire to be touching the surface of the track; that way there is as much friction as possible between the tire and the track. The tires on your car have treads that cut through mud and water to get to the nice firm road underneath. The treads actually give you less friction on a flat dry road!

Some of you might have used a skateboard shoe for this activity. Notice that the skateboard shoe has quite a flat bottom compared to most other shoes. This is because a skateboarder wants as much of his or her shoe as possible to touch the skateboard at all times.

See friction can be fun! See you next time with some more experiments.