Fast and Easy Science Fair Projects: Rub the Right Way

Friction and surfaces

Purpose: Compare the friction on a dry surface to one coated with oil.

Overview: Friction is the resistance to motion when two tings rub together. Friction is often undesirable. It makes machines less efficient where moving parts come in contact with each other. But there are times when friction is helpful. On the road, it's friction between a car's tires and the road's surface that allows a driver to keep control of the car. If a road ecomes covered with water, snow, ice or spilled oil, the car becomes harder to steer and to stop, this is especially true on a hill.

Hypothesis: Hypothesize that if friction becomes less, and object on a slope will need less of an angle for gravity to overcome friction.

You need:

• 2 pieces of wood (about 2 feet long (60 cm)
• 2 small plastic butter tubs with lids
• sand
• vegetable oil
• an old rag
• several books
• ruler
• protractor

Procedure: Make a ramp (the slope) using a piece of wood about 2 feet (60 cm) long and 3 to 4 inches (7-10 cm) wide (a 2-by-4 board works as well). To raise one end of the ramp, place several books under one end.

     Using an old rag, wipe some vegetable oil onto the board, coating and completely covering the surface. This represents spilled oil on a roadway.

     Fill two empty plastic butter tubs with an equal amount of sand, and close the lids.

     Place one of the filled tubs in the center of the board. By adding some more books or pushing them a little farther under the ramp, slowly make the ramp steeper until gravity overcomes the friction between the surfaces and the tub moves. When this happens, stand a ruler alongside the highest point of the ramp. Measure and write down the height of the ramp at that point, Then, using a protractor at the low end, measure the angle, or slope, of the ramp from the table or floor.

     Using books and another board, make a ramp with the same slope as the first ramp. Place the rub in the middle of the board. This time, the tub does not move. Slowly raise the slope of the ramp by adding more books until the tub finally moves. Measure the angle of the ramp, using a protractor, and see how much steeper it is compared to the first ramp. The weight here is now the Constant, and the surface friction is the Variable.

     What other places can you think of where friction is desirable? Think about walking on patches of ice outside, a newly waxed kitchen floor, or the tile floor in the bathroom when tou step out of the shower.

Results and Conclusion: Write down the results of your experiment. Come to a conclusion about your hypothesis.

Something more: Instead of comparing an oil-covered surface to a dry surface, compare a dry surface to one that is covered with ice. Place a piece of wood under the faucet in a sink and run water on it. Then put the wet piece of wood in the freezer and leave it there until the water has turned to ice. Again, find the angle where gravity overcomes resistance and sand-filled tub moves. Do you think driving a car on an ice covered road is more dangerous than when the road is dry? Besides driving more slowly, what do people do to help make driving on snow and ice safer?