# What Keeps an Object Moving Along a Circular Path?

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### The Idea

What determines how fast a car can safely go around a curve and not skid on the road? This project explores turning and friction, and how the two are related.

### What You Need

• board (approximately) 36 inches by 4 inches by ¾ inch (Other shapes, including a circularly shaped board or a turntable, can also be used.)
• vertical pole, such as a ring stand, to serve as a pivot point
• a few closely matched toy cars, such as Matchbox cars

### Method

1. Drill a hole in the center of the board. The hole should be large enough to allow the board to freely rotate on the post.
2. Place each of the cars along a line running from the center to the outer edge of the board at approximately 6-inch intervals, as shown in Figure 15-1. (You can also do this with pennies or other objects instead of cars.)
3. Predict what you think will happen to the cars as you start to rotate the board around the pivot point.
4. Rotate the board, very slowly as first, but then pick up speed. What happens to the cars?

### Expected Results

Cars furthest from the center begin to move first. As the cars start to move, they move away from the center, as shown in Figure 15-2.

### Why It Works

The cars remain on the board as long as the frictional force is greater than the centripetal force needed to keep the cars moving in a circular path. The further you are from the center of rotation, more centripetal force is needed. For this reason, the cars furthest from the center are the first to move.

### Other Things to Try

This can also be done using pennies on a rotating surface, such as a turntable.

### The Point

Friction can provide the centripetal force needed to keep an object moving along a circular path. If the force of friction is not sufficient to provide the centripetal force for a given radius, the object will depart from its circular path.
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