Volleyballs are useful in a variety of science fair projects, including pressure demonstrations and models of the solar system. A volleyball can represent the sun in a model of the solar system if the project creator paints it orange.
For a physics demonstration, a volleyball can demonstrate Newton's Law of Inertia, which explains the motion and interaction of objects. The demonstrator can throw the ball at objects of varying strength and rigidity, such as pillows and concrete walls. This demonstrates the ability of various objects to change the motion and speed of the ball.
Using a tennis ball with a volleyball can demonstrate how potential energy converts to kinetic energy. When the volleyball and tennis ball drop separately, they both bounce at approximately the same height, but when the volleyball drops with the tennis ball on top of it, the tennis ball bounces higher because it absorbs kinetic energy from the volleyball.
To demonstrate the optimal pressure of a volleyball, the demonstrator can inflate volleyballs at varying pressure levels and drop them from the same height. When dropping each volleyball, the demonstrator can point out the relation of bouncing height to the air pressure of each ball.
Another science fair idea is to paint the continents and oceans of the Earth on a volleyball so it resembles a globe.