Spinning Glasses of Water – 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

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Centripetal Force Board - Cover Image

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.  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.

 

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Here's What You'll Need

  • Rope
  • Water
  • Three plastic cups
  • Thin square board
  • Square sheet of rubber
  • Adult supervision

Let's Try It

  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. Centripetal Force Board - Step 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. Centripetal Force Board - Step 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

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. Even when the water is at the top of the circle, the force continues to accelerate the water downwards in the circular pattern, keeping it from spilling down on your head.

But what about gravity pulling the water downwards when it is at the top of the arc? One can explain that Force = Mass x Acceleration.  The force of the circular motion is greater than the force of gravity on the water, as long as the acceleration remains high enough. Therefore, the water doesn’t leave the bucket or glasses.

This demonstration provides the opportunity to discuss non-inertial (accelerated) frames of reference and inertial (fictitious) forces (such as the centrifugal force).

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