It’s not magic that keeps people in roller coaster cars that travel in looping, spiraling paths – it’s physics. Try this experiment to see how centripetal force and inertia keep people inside cars even when traveling upside down.

What You Need:

  • Small plastic bucket
  • Water
  • Sturdy string
  • Outdoor place that is OK to get wet

What You Do:

  1. Tie the string securely around the handle of a plastic bucket. Use a plastic bucket rather than metal so it is not heavy.
  2. Pour a glass full of water into the bucket. (Again, don’t add too much water so it is not too heavy.)
  3. Hold the string so that the bucket is about level with your knees. Adjust length of string as needed.
  4. Spin the bucket of water over your head in a vertical (up and down) direction. Make sure you spin the bucket fast enough so it stays in a circular path. Does the water spill out?

What Happened:

It seems as if the water in the bucket is defying gravity, but is it really? No. Gravity – the force pulling down on everything – is still at work even when the bucket and water are above your head. The water’s inertia wants to keep the water traveling in a straight path, but gravity is acting on the water, causing it to fall in a downward path that will eventually hit the earth. However, while the water is falling, the bucket is falling with it, catching the water. What keeps the bucket and water moving in a nice circular path that doesn’t get wet or messy is the string. The string acts as the centripetal force that pulls the bucket and water into the center and keeps them from following their paths of inertia, giving the illusion that centrifugal force is pulling the water away from the center. But be careful. In order for the bucket to keep falling with the water, the bucket must travel fast enough to keep up with the water. If you spin the bucket too slowly, the water will fall out and you will get wet.