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Bean Buoyancy

This “magic trick” reveals how relative densities cause objects to float or sink, even in a sea of solid beans. 

Objects with a density lower than the fluid that they are submerged in will float and objects with a greater density will sink.

In this set-up, the pinto beans form a coarse fluid. The metal ball has the highest density, the ping-pong ball has the lowest density, and the pinto beans have a density somewhere in between.

The fluid begins to flow when the container is shaken from side to side. The high-density metal ball sinks, while the low density ping-pong ball bobs up to the surface.

If shown as a magic trick, it appears that the metal ball turns into the ping-pong ball.

Objectives

  • Investigate the relative densities of familiar solids.

Materials

  • Per Demo or Group:
    1 table-tennis (Ping Pong) ball
    1 metal ball about the same size as the ping-pong ball
    1 bag of dried pinto beans
    1 large, opaque container
    1 large, clear container

Key Questions

  • What happened to the metal ball? Why?
  • What happened to the table-tennis ball? Why?
  • What would happen if a similar experiment were performed in water by dropping the metal balls from the top and releasing the table-tennis balls from the bottom? (The same effect happens in water because water has a density somewhere in between that of the metal balls and that of the table-tennis balls).

What To Do

Preparation

  1. Out of the students’ sight, put the table-tennis ball in the bottom of the opaque container.
  2. Pour the beans in the bowl, covering the table-tennis ball.

Part 1: The “Magic” Trick

  1. Invite students to find a space to watch your magic trick.
  2. Show them the container and say that it is full of magic beans.
  3. Lay the metal ball on top of the beans and explain that the magic beans will change the metal ball into something else.
  4. Shake the container gently. The metal ball will sink to the bottom, and the table-tennis ball will rise to the top, making it look like the metal ball has transformed.

Part 2: The Reveal

  1. Was it magic, or was it science? Explain to the students that you actually started with both balls. Ask them to try to explain how you performed this trick.
  2. Repeat the demonstration, this time in a clear container, showing the students the set-up.
  3. The table-tennis ball floats to the top of the beans because it has a lower density than the beans, but the metal ball sinks because its density is higher than the density of the beans.

Extensions

  • Experiment with objects of different densities using the beans as the fluid.
  • Explore the particle nature of matter and the similarities and differences between the beans and the particles of a liquid