Bean Buoyancy

What it shows

Objects with a density lower than the fluid that they are submerged in will float; objects with a greater density will sink. This is shown using a brass ball and ping-pong ball of equal size, and a sea of beans.

How it works

500g of navy beans form a rather coarse fluid in a 1.5L glass beaker. Embedded in the beans is a ping pong ball, and sitting on the surface is a brass ball, 4cm in diameter. This fluid needs to have flow 'induced', and this is done by shaking the beaker side to side. The ratio of densities of brass:beans:ping-pong is approximately 100:10:1; the high density brass ball sinks, while the low density ping-pong ball bobs up to the surface.

figure 1 brass ball turns into ping-pong ball!

before and after diagram of light and heavy balls


The bean buoyancy analog is cute but shouldn't be taken too seriously. The references below give details. It is however a neat party trick and the demo should have a 'street magician' delivery, otherwise it's a little half-baked.

photo of beaker full of beans

1. G. Spagna, Am J Phys 49, 507 (1981), "Buoyant force analog: A demonstration for the vertical stage overhead projector"
2. A. Rosato, K.J. Standberg, F. Prince, and R.H. Swendsen, Phys Rev Lett 58, 1038 (1987), "Why the Brazil nuts are on top: Size segregation of particulate matter by shaking"
3. R. Prigo, Physics Teacher 26, 101 (1988), "Liquid Beans"
4. R. Winter, Physics Teacher 28, 104 (1990), "On the Difference between Fluids and Dried Beans"
5. I. Peterson, Science News 143/26, 405 (1993), "Great shakes: Why pebbles wind up atop sand"