# Understanding Cars, Brakes, Friction and Gravity

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#### Updated on Dec 09, 2011

Grade Level: 6th to 8th; Type: Physics

### Objective:

This project determines how friction and gravity affect cars and braking.

### Research Questions:

• Do you think the front or rear brakes on a car lock first when the driver steps on the brake pedal?
• On older cars, that had brakes on only one pair of wheels, which pair of wheels, front or back, had the brakes?

### Materials:

• Toy car or truck (at least 4 inches in length, with wheels that turn freely)
• Long, wide, smooth board
• Rubber bands
• Pen or pencil
• Lab notebook

### Experimental Procedure

1. Place the toy car on one end of the board. Lift the board just until the car begins to roll. Calculate the slope of the incline where the car began to roll. To do this, divide the length of the board by the height of the incline. What does this tell you about the friction between the car wheels and the board? Record your data.
2. Now wrap a rubber band around both the front and back wheels so that none of the wheels can turn. Will the car slide down the incline now? If you raise the board higher, will the car move? If so, calculate the slope and record your data. How does the friction between the board and the locked wheels compare to the friction between the board and the wheels when they were free to move?
3. Next, remove the rubber band from just the front wheels, leaving them free to turn. Raise the board again until the car begins to move. Calculate the slope. What happens to the car as it slides down the hill?
4. This time, wrap the rubber band around the front wheels, but remove the rubber band from the back wheels, leaving them free to turn. What happens when you lift the board this time?
5. Analyze all of your data. In a real car, do you think the front or the rear brakes lock first when the driver applies pressure to the brake pedal? Older cars only had brakes on one set of wheels. Do you think the brakes were on the front wheels or the back wheels? Explain why.

Terms/Concepts: friction, motion, gravity, slope, incline

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