Phantastic Physics Demos: Levitating A Sphere
A great proof of Bernoulli's principle is to levitate a sphere in a funnel by
blowing down on the sphere. The top of a 2 liter soda bottle makes a perfect funnel and
the Styrofoam ball should be between 4 and 6cm in diameter. As a result of
Bernoulli's principle, it is possible (with enough air in your lungs) to keep the ball
from falling out of the funnel just by blowing down into it. A video of someone
levitating the ball is shown below.
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Here's the breakdown of why the ball can levitate:
- As the air tries to get around the ball it ends up moving very fast around the top
of the ball. Remember, faster moving air exerts less force on the surface it is
moving across.
- The air beneath the ball is moving very slow so it is exerting more force on the
ball.
- If the sum of the forces from the air are bigger than the weight of the ball then
it may be possible to keep the ball from falling. The harder a person blows, the faster
the air above the ball goes. The faster the air above the ball the greater the net
force pushing up on the ball.
- A similar situation can be set up using a garden hose connected to a large funnel
and when you turn the water on you can levitate a basketball!
- A spin off of this trick is to challenge someone to blow a ball out of the funnel
from below. See The Blow Out Challenge.
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