Bernoulli's theorem states that an increase in the speed of the fluid occurs simultaneously with a decrease in pressure. In other words the velocity increases pressure decreases.
Bernoulli's Theorem allows us to understand why planes can fly for example.
We can explain this theorem using available materials and very simple demonstrations.
What we need:
- ping pong ball;
- cardboard tube, look in kitchen for paper towels rolls.
- Turn on the dryer;
- Select the maximum speed and the lower temperature;
- Point the dryer up;
- Place the ball in the airflow, find the equilibrium, this may require some practice;
- When the ball is stable in the air, approach the tube from the dryer.
The ball is sucked through the tube and comes out the other side.
Because the force balance is disturbed.
Bernoulli theorem states that a liquid or a gas lose pressure when its velocity increases. In this demonstration the ball was steady in a cold air flow, the air was moving and the pressure was low. The air exits the dryer and moves around the ball. This system is in equilibrium and the pressure is kept constant. This allows the ball to be in place.
When we add the tube to the system, we force the air to pass thru it, and therefore circulating in a confined space. This leads to the increase of air velocity, and consequently to the pressure decrease inside the tube, and that’s why the ball is sucked in the tube, in an attempt to recover the system balance.
Check out the video:
Connolly, Sean, 2008 "The book of totally irresponsible science", Workman Publishing, NY