What you need:
A hair dryer
A ping pong ball
An empty toilet paper tube (through which the ball can pass)
What to do:
1. Switch on the hair dryer and point its nozzle upwards to the ceiling.
2. Place the ping pong ball in the jet of air being released by the hair dryer. Wait until the ball is steady in the stream of air.
3. Hold the toilet paper tube ready over the ball. Then, slowly, start lowering the tube towards the ball.
What happens:
When you lower the toilet paper tube towards the suspended ball, there comes a point when the ball slips into the tube. Then it quickly moves through the tube and falls out, down to the ground.
Why?
According to Bernoulli’s principle, the faster the air flows over a surface, the less it pushes on the surface i.e. the pressure exerted by the air on that surface decreases.
The hair dryer shoots out fast-moving air. When the ping pong ball is placed in the path of this fast-moving air, the air molecules exert low pressure on the ball because they are too busy shooting up really fast. But the air above, the ball is not moving much. So it has higher pressure. This high pressure or push doesn’t allow the ball to shoot up and away. Thus the ball holds steady.
Now, let’s meet Giovanni Battista Venturi, an 18-century Italian scientist, who came up with something known as the venturi Effect which is related to the Bernoulli Principle. Venturi found that if fast-moving air (or any moving fluid) is restricted, i.e. it is forced to flow through a narrow or choked space (in this case, the toilet le if it paper tube), the air speeds up even more and the pressure it exerts decreases further.
Sometimes, when fees, when you are standing at the foot of a skyscraper, you find that the wind there is really fast. This mostly happens when skyscrapers are built close together. Air speeds up when it has to squeeze through the small separation between the tall buildings. That’s Venturi Effect.
Accordingly, the air from the dryer speeds up when it enters the tube, and its pressure falls. This creates a low-pressure zone above the ball (when the tube is above it). That means the air below the ball pushes against it more than the air above the ball. That’s how the ball slips into the tube and then out. Once it is out, it has already left the steady upward jet of air from the dryer, which is why gravity can pull it down and ping pong, it goes to the ground!
Picture Credit : Google
