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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: The Blow Out Challenge

A great proof of Bernoulli's principle is to challenge someone to blow a Styrofoam ball out of a funnel.  The top of a 2 liter soda bottle makes a perfect funnel and the ball should be between 4 and 6cm in diameter.  As a result of Bernoulli's principle, the challenge proves to be close to impossible.  A video of someone taking the challenge is shown below.

 

animation of air stream Here's the breakdown of why the ball is nearly impossible to blow out:
  • As the air tries to get around the ball it ends up moving very fast around the base of the ball.  Remember, faster moving air exerts less force on the surface it is moving across.
  • The air above the ball is moving very slow so it is exerting more force on the ball.
  • The net force on the ball is down, and the harder a person blows, the faster the air under the ball goes.  The faster the air under the ball the greater the net force pushing down on the ball.
  • The difference in force can be so great that if you turn the funnel upside down while blowing through it, the ball may not fall out. (need good lungs for this).  See Levitate a Sphere.