I spent several weeks exploring different ways to get balls to float on a stream of air. The goal was to find ways to show Bernoulli's Principle and the Coanda Effect in action. Now, the Floating Balls demonstration is an important piece of my presentations at schools were I engage kids with "The Magic of Science". Let's first explore the demonstrations and then we will look at the science behind the "Magic Floating Balls"!
How to Make Magic Floating Balls
Note: Because this is a physics demonstration (see explanations below), you must understand that the force of the air moving upward must overcome the force of gravity pulling the ball downward - this means you will have to find a ball and a blower that are a good match.
Point the stream of air (leaf blower, hair dryer, etc) upward and gently place a ball into the stream of air. The ball should hover (as long as it is the right size and weight for the stream of air you are using). Then tilt the stream of air from side to side to model Bernoulli's Principle and the Coanda Effect.
Explanation of the Magic Floating Balls Demonstration
This demonstration has a lot of simple and complex physics concepts. First we need to understand forces. The stream of air (we will call this our "jet") provides a force on the ball. The direction of the force depends on what direction the jet is facing. If we face the jet upwards, the force "pushes" the ball upward. Gravity is also acting upon the ball and it provides a force that pulls the ball downward (toward the earth). When these two opposing forces are equal, the ball hovers in the jet of air. But, why doesn't the ball just fall off to the side of the jet and then fall to the earth? Let's start with Bernoulli's Principle.
What is Bernoulli's Principle?
Bernoulli's Principle states that an increase in the speed of a fluid (in this case a jet of air) causes a decrease in the pressure of the air. And because air flows from areas of high pressure toward areas of low pressure, you end up with some interesting pressure dynamics around the ball.
Let's think about our spherical ball. As the air hits the ball, aerodynamics cause the air to move around the outside of the ball. As it does, the space where the air is moving right next to the ball is constricted (there is less space for it to move) This constriction on all sides forces the air to move at a different speed (it speeds up slightly) than the air around the ball that is not directly touching the ball. This increase of speed causes an area of low pressure all around the outside of the ball. This even pressure around the outside of the ball holds the ball in place - it 'holds' it in the jet of air. But, Bernoulli's Principle does not explain the whole story. In order to fully understand what is happening, we must understand the Coanda Effect. (More info)
What is the Coanda Effect?
The Coanda Effect explains that a jet of fluid (in our case a jet of air) has a tendency to stay attached to a convex surface (our ball is a convex object). This physics principle helps us understand why the ball stays in the jet of air. If you watch the video carefully you will see that the balls wobble in the jet. This is caused by a lot of external factors (wind, movement of the blower, momentum, etc). As the ball starts to move out of the jet, the Coanda Effect explains that the jet of air on all sides 'adheres' to the ball, thus pulling it back into the center of the jet.
The Coanda Effect is best modeled when the jet is held at an angle and the ball stays in the jet. The fast moving air "sticks" to the ball and holds it in place in the lower pressure zone of air! (More info)
Bernoulli's Principle and the Coanda Effect are both explanations that help us understand how airplanes fly. Because an airplane is moving quickly through the air the same principles apply because if we were sitting in the plane we could pretend we were sitting still and the air is moving quickly past the airplane (Newton's Theory of Relativity). As the plane moves through this steam of air, the pressure near the wings changes (Bernoulli's Principle) and the wings stick to the fast moving air to keep the plane in the air (Coanda Effect).
This is how we get a magic floating ball!
But, now we know that it isn't actually magic that is keeping it afloat, it is the principles of science!
Keep on Learning! Craig Beals