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.
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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.
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