Recent visitors to the Portland Art Museum were treated to a viewing of the work One Ball Total Equilibrium Tank by the internationally renowned artist Jeff Koons. As visitors admired the piece, they often wondered how the basketball was being suspended in the center of the tank without wires or other visible means of suspension. Was it an optical illusion? A magic trick? No, it was simply the intersection of art and science—a study in equilibrium and buoyant force. With dense, highly saline water in the bottom half of the tank, and less dense distilled water on the top, a basketball of just the right density is held in place by opposing forces.
Your students can easily recreate this phenomenon in the classroom. A large graduated cylinder and golf ball work well, but the options are unlimited. To quantify the forces involved, a sensor such as our new Go Direct® Force and Acceleration Sensor can be used.
When we investigated this phenomenon, we partially filled a graduated cylinder with a saturated solution of sodium chloride in water and then very gently added fresh water. Because of the significant difference in density, these two layers remained stratified with only a few vertical centimeters of mixing. Data were collected as the golf ball, suspended from the Go Direct™ Force and Acceleration Sensor, was lowered through the column of water. The data show that as the ball passed through the fresh water, the net force remained fairly constant. As the ball reached the salinity gradient, the net force quickly decreased to 0 N and the ball ultimately stopped its descent. Resting at equilibrium between the two layers of water, the ball was suspended, just like Jeff Koons’ iconic basketball.