Make a ball hover

Try putting a light ball (ping pong balls work well) into a stream of air blowing straight up from a hairdryer. (Make sure it’s on a cool setting!) Once the ball ‘balances’ try lowering the angle of the stream of air.


What happens to the ball?

What angle you can lower the stream of air before the ball falls?

An object can be pushed up or down by the movement of the air around it. If the lift upwards is more than the weight of the object (and any other forces) pulling downwards, then it will be able to fly!

You can experience lift yourself. Place a strip of paper below your mouth, and blow over it. You will notice that the paper rises.

18th Century mathematician, Daniel Bernoulli, was interested in blood flow. (He was also a medical doctor!) He showed that as the speed of a fluid increases, the pressure it exerts decreases. This is true for air. The faster-moving air over the top of the paper is at a lower pressure than the still air underneath. This means it pushes the paper down less than the air underneath pushes it up and so the paper rises. The movement of air around an aeroplane is a bit more complicated. The angle of the wings cause large volumes of air to be deflected downwards and swirling air currents known as vortices are created. However the overall effect of all this fluid flow is that the pressure above the wing is lower and the plane experiences lift.