Spin the Bucket: A Centripetal Force Experiment
Have your middle schooler try this classic and entertaining activity that demonstrates circular motion. Due to the nature of flying water and bucket, it is best to perform this activity outside. Be careful, you may get soaked!
What You Need:
- Small bucket with a handle
- Rope (about 3 feet long)
What You Do:
- Tightly tie one end of the rope to the bucket handle.
- Fill the bucket about half-full with water.
- Have your child wrap the other end of the rope around her hand one or two times. (Once the bucket is spinning, the rope will pull on her hand, so there is a fine line between being too tight and too loose.)
- Make sure your child is far enough away from other people that no one can get hit by a flying bucket of water! Have your rocket scientist spin the bucket in a vertical circle fast enough that the water does not fall out of the bucket.
- Once it has been established that water will not fall out of the bucket, have your child slow (not stop) the motion of the spin so that the water begins to “splash”. How slow can she spin the bucket before water begins to fall out?
- Clear the way! Have your child spin the bucket fast again and release the rope when the bucket is at the lowest point of the circle. Observe the flight path of the bucket.
- Challenge your child to think of things that take advantage of this scientific phenomenon. Consider satellites, amusement park rides, and household appliances.
What is going on here?
The spinning bucket of water is a classic demonstration of "centripetal force", the inward force acting on a rotating object. Ask your child if the water is falling while it is spinning in the bucket. The answer is yes: it just happens that the bucket is falling just as fast as the water. Imagine that the bucket disappeared at the height of the circle: the water would not just “hang” in mid-air, it would fall to the ground! This is what happens as the spinning bucket slows enough to allow the water to splash; the water begins to fall faster than the bucket is falling. Slow the motion of the bucket enough and the water falls out all together.
A satellite orbiting earth is a real-life example of the spinning bucket demonstration. The satellite is constantly “falling” around the earth, but since the earth is curved, as long as the satellite is moving fast enough, it does not fall to the earth’s surface; rather, it continually falls around the earth.
When your child let go of the bucket, it should have continued in a straight line away from her. This is an example of Newton’s First Law of Motion at work (an object in motion continues in a straight line unless acted up by an outside force). Ask your child which direction the bucket would have flown if she let go as the bucket was swinging upward. It would continue in a straight upward motion (I don’t recommend doing this, as what goes up, must come down!).
Challenge your child to one last question: If according to Newton’s First Law, the bucket will continue in a straight line, what causes the bucket to fall? Good old gravity.
Who knew that there was so much science involved in swinging a bucket?