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{\bf Question}
Use Bernouilli's law to explain why a ping-pong ball can be
supported by an upward jet of air.
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{\bf Answer}
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Bernoulli's law along the particle path shown gives $ p +
\frac{1}{2} \rho u^2 = p_A,$ (RHS because evaluating at A) where
$u$ is the speed of flow.
Therefore $p = p_A - \frac{1}{2} \rho u^2 $ (*).
As the fluid moves away from $A$ its speed $(u)$ increases and
hence from (*) $p$ decreases. Thus the pressure is greater on the
lower half of the ping-pong ball than the upper half. Pressure
acts normally on the ping-pong ball and so there is a net upward
pressure force that can oppose downward gravity. As a consequence
of pressure acting normally there are net opposing lateral
condition on each side that ensures the ball is \lq trapped'.
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