Spin the Bucket: A Centripetal Force Experiment

What You Need:

  • 3 foot rope
  • Water
  • Little bucket with handle

What You Do:

  1. Tie one end of the rope to the handle of the bucket.
  2. Pour water into the bucket so that it's filled halfway.
  3. Wrap the rope's other end around your child's hand a couple times. Make sure it's not too tight or too loose, as the bucket will pull on the rope.
  4. Invite your child to rotate his hand to quickly spin the bucket in a vertical circle so that water doesn't fly out. Ensure that there's enough space for him to move the bucket.
  5. Ask your child to then rotate the bucket slowly so that water begins to splash around. Notice how slowly the bucket has to spin for water to fly out!
  6. Encourage your child to rotate the bucket quickly and let go of the rope when the bucket is at the bottom of the circle. Where does the bucket go? Make sure the area is clear when your child does this!
  7. Ask your child what everyday things incorporate circular motion. What about roller coasters?

What's Happening?

This activity is an example of centripetal force, which is when an inward force acts on a circular-moving object. In this bucket activity, the water is falling at the same speed as the bucket, so the liquid doesn't appear to fall out. If the bucket were to be removed or slow down at the top of the circle, the liquid would pour out because the forces are no longer balanced!

One everyday example of this phenomenon is a satellite. The earth is curved, so the satellite orbits quickly enough to fall around the earth. If it moved any slower, it would fall onto the earth.

Wait! There's more science! What happened when your child let go of the rope? Did the bucket move in a straight line? Newton's First Law of Motion plays a role here. This law states that something that's moving will keep moving in a straight line unless some other force acts on it and changes the direction. Challenge your child and ask him where the bucket would've gone if he had let go of the rope at other points in the circle.

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