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# Static Friction

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Author: Janice VanCleave

### Force That Resists Motion

Friction is a force that resists motion and occurs whenever anything moves while in contact with anything else. Everyday actions such as sweeping, brushing your teeth, and turning a doorknob all involve friction. Without friction you couldn't hold on to the broom, toothbrush, or doorknob. Your hand would slip and slide the way your shoes do on ice. Friction also results in the slowing of moving objects. A thrown baseball or Frisbee rubs against air as it moves, and the friction of this contact slows its motion.

In this project, you will measure the static friction (force needed to move an object) of an object. You will discover the difference between an object's static friction and sliding friction (force needed to keep an object moving at a uniform speed). And you will discover which of these types of friction is greater. You will also calculate different types of coefficient of friction (ratio between the force of friction and the weight of the object being moved).

### Getting Started

Purpose: To measure the static friction of an object.

### Materials

• 6-by-12-inch (15-by-30-cm) piece of cardboard
• transparent tape
• 9-ounce (270-ml) plastic cup
• 16-inch (40-cm) piece of string
• rubber band (a #1, medium size, band works well)
• scissors
• metric graph paper with 1-cm squares
• 20 marbles
• pencil

### Procedure

1. Place the cardboard on a table.
2. Tape the cup to the center of one of the shorter sides of the cardboard near the edge.
3. Tie the string around the bottom of the cup. Then tie one of the free ends of the string to the
4. rubber band. You want the rubber band to be as close to the cup as possible. Cut off the excess ends of the string.
5. Lay the graph paper on the cardboard, with one edge next to the cup. Secure the paper to the cardboard with tape.
6. Add the marbles to the cup.
7. With the point of the pencil inside the loop of the rubber band, pull the rubber band straight but do not stretch it.
8. With the rubber band in this position, make a mark on the graph paper. Then move the pencil in a straight path away from the cup, stretching the rubber band and tracing a line on the graph paper with the pencil point (see Figure 52.1).
9. Stop moving the pencil as soon as the cardboard moves forward.
10. Count the squares that your pencil crossed. For partial squares, estimate the fractions. Record the number of squares in a Friction Data table like Table 52.1.
11. Repeat steps 7 through 9 four times. Average the results of all five tests, and record the average in the Friction Data table.

### Results

You will be able to stretch the rubber band some distance before the cardboard moves. The exact results will depend on the elasticity of the rubber band as well as the surface of the table and the weight of the cup.

### Why?

Friction is the name of forces that oppose the motion of one surface relative to another when the two surfaces are in contact with each other. Friction acts parallel to the surfaces in contact and in the opposite direction of motion. Friction is due to the fact that no matter how smooth the macroscopic (large enough to be seen with the naked eye) view of a surface, its microscopic (so small it requires a microscope to be seen) view is rough. The irregularities on both surfaces that are rubbing against each other interlock and offer resistance to motion. Static friction is the force that opposes the start of motion of an object.

In this investigation, the distance the rubber band stretches (length of line in number of squares) indicates the force of static friction between the surface of the cardboard and the surface of the table. The more the rubber band stretches before the cardboard moves, indicated by the number of squares crossed by the line, the greater the force of static friction.

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