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In support of all the students who are displaced from school due to the Corona virus. Access to physics zone and chemistry zone lessons are now available free of charge. This will be maintained at least through August 1st 2020. Learn and be well.

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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.

 

funnel forces 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.