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Centripetal Force Board

Swinging an open glass of water sounds like a recipe for a mess, but not when you understand the physics behind the trick!

When you swing a bucket of water over your head, you probably expect a big, wet rush of water to soak you as the bucket goes upside down. However, if you were swinging the bucket fast enough, nothing happens. What in the world?!? Well, we have a pretty incredible physics demonstration to help you understand just why you remain dry. It's called the Centripetal Force Board, and it will help you experience the physics and forces at play when three plastic cups of water leave you high, dry, and with new hands-on science topics in mind.

Experiment Materials

Experiment

  1. Drill a hole large enough for the rope in each corner of the board.
  2. Glue the thin sheet of rubber to one side of the board.
  3. Tie the center of two even lengths of rope together.
  4. Pull one strand of rope through each hole and tie the ends. Hold up the board to make sure the ropes are even.
  5. Place the 3 plastic cups in the center of the board and fill them with water.
  6. Slowly begin swinging the board and when you are ready, swing it in a complete circle. Then, slowly bring the board to a stop.

How Does It Work?

Here's the heavy-duty science for people who really care…

According to Newton's First Law of Motion, objects in motion tend to remain in motion unless acted upon by an external force. In this case, Newton's Law requires the water to continue moving along a tangent to the circle. Thus a force is required to keep it always turning toward the center of the circle. The interpretation of this demonstration is potentially confusing when one considers that at the top of its arc, the water is accelerating downward because of the motion, but that the force of gravity is also downward. One can explain that F = ma is thus satisfied without the water leaving the bucket. This demonstration provides the opportunity to discuss non-inertial (accelerated) frames of reference and inertial (fictitious) forces (such as the centrifugal force).