Winding up the spring (6 to 8 turns will do quite nicely) provides the power for the car. When it is turned on the car moves slowly across the table while producing a vertical air stream which supports a small styrofoam ball. The ball levitates in the upward air stream due to the Coanda effect, causing the ball to follow along with the car. This demonstration is often explained incorrectly using the Bernoulli effect.

According to the INCORRECT explanation, the ball (or balloon or beachball, etc.) positions itself at the edge of the moving air, with the inside part in the rapidly moving air stream and the outside in the quiescent adjacent air. The pressure is lower in the moving air jet, so the differential pressure keeps the ball levitated in the air stream. The correct explanation involves the Coanda effect. When the air stream flows past the ball, some of the air follows the contour of the ball and only leaves after it moves a significant distance along the surface of the ball, as illustrated in the drawing below. In effect, the ball is "pulling" the air around its surface. There must always be some reaction force on the ball, which points in the direction of the air stream and upward, holding the ball in the air.

Be CAREFUL! Car is fragile and not replaceable.

A slow cart moves either horizontally or at an angle while creating an air stream which supports a styrofoam ball.

The ball levitates in the upward air stream due to the Coanda effect, causing the ball to follow along with the car.

This demonstration is often explained incorrectly using the Bernoulli effect. According to the INCORRECT explanation, the ball (or balloon or beachball, etc.) positions itself at the edge of the moving air, with the inside part in the rapidly moving air stream and the outside in the quiescent adjacent air. The pressure is lower in the moving air jet, so the differential pressure keeps the ball levitated in the air stream.

The correct explanation involves the Coanda effect. When the air stream flows past the ball, some of the air follows the contour of the ball and only leaves after it moves a significant distance along the surface of the ball, as illustrated in the drawing below. In effect, the ball is "pulling" the air around its surface. There must always be some reaction force on the ball, which points in the direction of the air stream and upward, holding the ball in the air.

This is demonstrably incorrect, as can be seen in the videos below, where one of the sheets is held away from the other and the air stream directed as in the above video. Note that the remaining sheet that is hanging moves toward the center even in the absence of the other sheet, so it does not form a narrow constriction. What is happening here is that the air moves along the surface of the sheet, according to the Coanda effect, leaving in a direction away from the center line of the two sheets. The reaction force on the sheet causes it to move toward the center line of the two hanging sheets. Click your mouse on the photographs below to see these two demonstrations of the Coanda effect.

The correct explanation involves the Coanda effect. When the air stream flows past the ball, some of the air follows the contour of the ball and only leaves after it moves a significant distance along the surface of the ball, as illustrated in the drawing below. In effect, the ball is "pulling" the air around its surface. There must always be some reaction force on the ball, which points in the direction of the air stream and upward, holding the ball in the air.

Invite students to compare this to how the chimney of a heating system works.

The nature of the actual lifting force on a real airplane wing is complex. See the Demonstration Reference File for several papers which discuss this problem. One can argue that the Bernoulli effect creates a pressure difference between the top and the bottom of the airplane wing. However, this pressure difference, in the absence of air deflection downward cannot explain the lift required to keep the airplane up in the air. According to Newton's third law, there must be deflection of the air downward due to either or both: deflection of the air due to the angle of attack of the wing, and shedding of vortices at the trailing edge of the wing.

An alternative explanation of airplane wing lift involves the Coanda effect and downward deflection of the air passing over the wing. According to the Coanda effect, the air flow follows the contour of the wing, ultimately moving at an angle downward from the rear of the wing. The reaction force acting on the wing provides the wing lift.