Resource: Defy Gravity! Balancing Balls on Air

Liquids and gases are both considered fluids for certain purposes of analysis. Fluids are substances whose molecules, unlike those of solids, do not occupy fixed positions at normal temperatures because the forces that bind them together are relatively weak. From experiments conducted in the eighteenth century, the Swiss mathematician Daniel Bernoulli determined that for a fluid in motion, pressure and velocity are inversely related: pressure is greatest when speed is lowest, and vice versa.

By applying Bernoulli's principle, aeronautic engineers have been able to design airplane wings that can help put 400-ton passenger jets in flight and keep them there. The top surface of the wing is curved and the lower surface is flat, and as a result, the air rushing over the wing, which has a longer distance to travel, has a greater velocity than the air passing under the wing. Because pressure is greatest where velocity is least, the pressure pushing up on the wing from below is greater than that pushing down from above. The difference in pressure provides a net upward force, called lift, on the wing.

We can see Bernoulli's principle in action in this demonstration adapted from ZOOM. The air blown up from the hair dryer is of sufficient velocity and volume to counter the downward force of gravity acting on the balls. Because of the fairly random molecular movement of fluids, the balls don't stay still. But unless the airflow is interrupted, they also don't fly away. What keeps the balls floating within the air column, constantly adjusting back to the center, can be explained by Bernoulli's principle.

If a ball begins to drift to one side of the air column, it quickly returns to the center. This is because the air around the center column of fast-moving air is moving more slowly and therefore has a greater pressure. When the ball drifts outside the center of the air column and into a region where the air is moving more slowly, the greater pressure in that region pushes the ball back into the fast-moving air column. Even when the cast members try to intentionally direct the airflow to one side or the other, the ball always returns to the fast-moving center of the air column.

What force pushes the ball up? What force pulls the ball down?

What force keeps the ball in the air column over the hair dryer?

If you watch closely, does the ball seem to try to move sideways and get pushed or pulled back? Why doesn't the ball just slip sideways and fall off the air that is holding it up?

How can you make the ball fall sideways without touching it?

When two balls are balanced in the same stream of air, why do you think they continuously change position and float at different distances from the hair dryer?