Centripetal force is the force that makes you go in a circle. Newton’s First Law tells us that objects in motion will continue to move at a constant velocity in a straight line unless acted on by a force. The centripetal force is directed toward the center of the circle, changing the object’s direction. Instead of going straight, the object moves in a circle.

Think about when you ride a roller coaster. At the top of a hill, you may lift out of your seat. This is because the car is changing direction, but your body continues in a straight line. When your seat belt pushes you down, it applies a centripetal force making your body move in a curve with the track.

 Want to try this experiment at home or in your classroom? Watch the video for an overview, gather the materials listed at the right, and follow the instructions below!

Required Materials

  • cup
  • string
  • washer or coin
  • water

Step-By-Step Instructions

Step 1

Punch two holes near the top rim of the cup opposite each other and cut a length of string about twice the length of your arm.

Step 2

Thread the string through the holes in the cup and tie it in a loop.

Step 3

Place your coin or washer in the cup and swing it in a vertical circle like in the video. The coin should not fall out of the cup. It make take a couple tries to accomplish this.

Step 4

Now put a little water in the cup and try swinging it again. The water should not spill.