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# Static Friction (page 2)

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Author: Janice VanCleave

### Try New Approaches

1. How does the smoothness of the surfaces affect static friction? Repeat the investigation, placing the cardboard on different surfaces, such as waxed paper and different grades of sandpaper, that have been secured to the table.
2. How does the force pressing the surfaces together affect static friction? Repeat the original investigation, increasing the weight on the cardboard by adding more marbles to the cup.
3. How does lubrication affect static friction? Repeat the investigation twice: First use a surface of sandpaper alone. Then use sandpaper covered with a thick layer of petroleum jelly.

### Design Your Own Experiment

1.
1. Design a way to measure the static friction of a system. One way is to attach one end of a string to a small box and the other end to a paper cup. Set another cup filled with marbles in the box, and place the box on a table so that the empty paper cup hangs over the edge of the table. The box should start out about 6 inches (15 cm) from the edge of the table. Add weights to the empty hanging cup until the box system (box and contents) starts to move. This weight equals the static friction (Ff) of the system. For weights, use something like coins, paper clips, and/or washers that you know the individual weight of. Ask your teacher or a pharmacist to weigh whichever materials you choose on an accurate scale.
2. Friction between any two surfaces can be measured by the coefficient of static friction, which is the ratio between the force of static friction between two surfaces in contact with each other and the force holding the surfaces together. The coefficient of static friction is a constant that depends on the nature of the surfaces in contact with each other. Determine the coefficient of static friction between the cardboard and the material on the table's surface using this equation:
μ = Ff / FN
3. In this equation, μ (mu) is the symbol for the coefficient of static friction when Ff is the static friction (the total weight in the hanging cup) when the box moves, and FN is the perpendicular force pushing the surfaces together (on a horizontal surface, the weight of the box and its contents). For information about the coefficient of static friction for common surfaces, see a physics text.

2.
1. Sliding friction is the frictional force between objects that are sliding with respect to one another. Using the materials from the previous investigation, start by removing about half of the weights from the hanging cup. Then slowly add the weights to the hanging cup one at a time, but this time after each addition, give the box a slight push toward the edge of the table where the cup is hanging. Continue this process until the box starts to move at a uniform velocity (speed and direction of a moving object). (If the box accelerates—changes in velocity per time—the force is too large; if the box stops, the force is too small.) How does the sliding friction compare to the static friction of the box?
2. How does the contact area of surfaces affect sliding friction? Design a way to measure the sliding friction of a system in which only the surface area changes. One way is to replace the box with a block of wood with different-size faces. Then determine the sliding friction of the wood for each of its different-size faces.
3. Determine the coefficient of sliding friction (the ratio between the force of sliding friction between surfaces in contact with each other and the force holding the surfaces together) for the box and table surfaces using the equation μ = Ff / FN, where μ (mu) is the symbol for sliding friction when Ff is the sliding friction.

### Get the Facts

1. You can move one object across another without sliding, and thus without sliding friction, using rollers. What are ball bearings and, when placed between surfaces, how do ball bearings minimize sliding friction? For information, see Louis A. Bloomfield, How Things Work: The Physics of Everyday Life (New York: Wiley, 1997), pp. 57–60.
2. Brakes on a car work because of friction. Why are brakes generally designed to be applied to the front wheels before the rear wheels? For an exploratory investigation to discover the answer to this question, see Robert Gardner, Experiments with Motion (Springfield, N.J.: Enslow Publishers, 1995), pp. 63–67.
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