Balancing Ball

Suspend a ball in a stream of air

A ball stably levitated on an invisible stream of air is a dramatic sight. When you try to pull the ball out of the airstream, you can feel a force pulling it back in. You can alsofeel that the airstream is being deflected by the ball. This activity shows one of the forces that give airplanes lift.

Small Ball   A hair dryer (blower)  A spherical balloon or table tennis ball   Tissue paper   Optional: a stand for the blower Large Ball   A vacuum cleaner It should come with a reversible hose, like a Shop Vac has, so it can be used as blower.   A light-weight vinyl beach ball   Tissue paper   Optional: A stand for the hose

Blow a stream of air straight up. Carefully balance the ball above the airstream. Pull it slowly out of the flow. Notice that when only half the ball is out of the airstream, you can feel it being sucked back in. Let go of the ball and notice that it oscillates back and forth and then settles down near the center of the airstream. With one hand, pull the ball partially out of the airstream. With the other hand, dangle a piece of tissue paper and search for the airstream above the ball. Notice that the ball deflects the airstream outward. On the large version of this activity, you can actually feel the deflected airstream hit your hand. Tilt the airstream to one side and notice that the ball can still be suspended. Balance the ball in the airstream and then move the blower and the ball toward a wall (try the corner of a room). Notice the great increase in height of the suspended ball.

When the ball is suspended in the airstream, the air flowing upward hits the bottom of the ball and slows down, generating a region of higher pressure. The high-pressure region of air under the ball holds the ball up against the pull of gravity. When you pull the ball partially out of the airstream, the air flows around the curve of the ball that is nearest the center of the airstream. Air rushes in an arc around the top of the ball and then continues outward above the ball. As the air arcs around the ball, the air pressure on the side of the ball nearest the airstream decreases, allowing the normal atmospheric pressure of the calm air on the other side of the ball (which is a higher pressure) to push the ball back into the airstream. An explanation is provided by the Bernoulli principle. The airstream speeds up as it flows around the ball. This is because the middle of the ball sticks farther into the airstream than the top or bottom. Since the same amount of air must flow past all parts of the ball each second, it must flow faster where it is pinched together at the middle. The Bernoulli principle states that where air speeds up, its pressure drops. The difference in pressure between the still air and the moving air pushes the ball back into the center of the airstream.

Follow-up 1. What holds the ball up against the pull of gravity? . 2. Describe the flow of air as it moves around the ball. Explain the process that holds the ball inside the air stream. 3. According to the Bernoulli principle, what happens to the speed of the airstream as it flows around the ball? Explain. 4. Why does air flowing over a surface in an arc exert less pressure on that surface? 5. If air flowing at a constant speed is suddenly pinched together what happens to its pressure? 6. We know what prevents the ball from escaping to the sides, but what downward force keeps the ball falling back into the air stream?