Bernoulli's Principle

What it shows

The relative velocities of two sides of a spinning ball to an oncoming wind creates a pressure difference and therefore a net force on the ball perpendicular to the air flow.

figure 1. Direction of motion of ball due to pressure difference

diagram of air flow and spinning ball

How it works

The air flow is produced by a Leybold wind generator. 1   The target of the wind is a styrofoam ball that is spun about its vertical axis on the end of an aluminum rod. It is driven by a 6V DC motor mounted at the base of a casing made of 2mm thickness aluminum. Two bearings ensure as little wobble or precession as possible. This assembly is held on two bearings inside a fork mount that is clamped to the bench. The ball/motor assembly is therefore free to swing in the plane perpendicular to the flow of air. The fork mount is made of 5mm thickness aluminum; the cut-away side view in figure 2 shows how the base of the fork mount has been shaped to give clearance.

When the ball is spun up in the air stream, the face moving into the wind has a lower relative velocity so a higher air pressure. The face moving away has a higher relative velocity and consequently lower air pressure. This results in the ball tipping over in the direction of low pressure, as in figure 1. Reverse the direction of spin to tip the ball the other way.

Setting it up

The Leybold blower is placed horizontally on a cart with the ball assembly clamped to the end of the cart about 1m away. Use a 6V DC power supply.


Good demo to explain sporting phenomena such as top spin in tennis, curve balls in baseball or a curving shot in soccer.

figure 2. Three views of the styrofoam ball assembly.

technical drawings of spinning axis assembly

1 Leybold wind tunnel available from Central Scientific, Cat. No. 37304

photo of blower and spinning ball