The nozzle makes the water flow through the upper part of the table tennis ball. If the angle and velocity of the water jet are well controlled, the table tennis ball will rise to the sky. The string is used to hold the table tennis ball and prevent it from being washed away by water. Someone told me that it's because of Bernoulli's law, because the water above table tennis is is moving, and the air below table tennis is is static. But I don't believe that explanation. Because of the relativity of movement, I can also say that the air under table tennis is is moving, while the water is static.
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Possible duplicates: https://physics.stackexchange.com/q/69731/2451 and links therein. – Qmechanic Mar 22 '19 at 11:27
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@Qmechanic See my answer to your question? – enbin Apr 06 '19 at 02:04
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It is the combination of two effects: Newton's $\dot{m_1}\overrightarrow{v_1}= \dot{m_2}\overrightarrow{v_2}$ , conservation of momentum. And Bernoulli's law.
The water stream washing off the top of the table tennis ball gets narrower around the ball and also deflects down.
By Bernoulli law the presser is reduced in the back of the ball because water moves faster. And by deflecting down because of viscosity, we have a $\dot{m} \overrightarrow{v} $ change downward, which creates equal and opposite and upward momentum in the ball. Similar to an airfoil.
kamran
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It seems that Bernoulli's law is wrong to explain it. Because force is the cause of the change of speed,there must be an increase in pressure first,and then there will be an increase in speed.The pressure increases and the ball goes down,not up. Can't it deflect without viscosity?think the real reason for the deflection is atmospheric pressure,not water viscosity. – enbin Mar 22 '19 at 22:50
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@enbin zheng, the impact of the stream will mostly by adding to the speed of the ball along. So I still think Bernoulli effect is helping. I have seen in creeks when there is a structure protruding into the path of water, like a concrete pier, downstream low pressure has created cavities and removed material away. As to deflection of the water due to viscosity i see your point, but in the situations where there is no air like a boat's rudder it happens because of viscosity. – kamran Mar 23 '19 at 00:00
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Because atmospheric pressure acts on the surface of the water, the pressure at the bottom of the water contains atmospheric pressure. Water Pressure + Atmospheric Pressure = Bottom Pressure – enbin Mar 23 '19 at 08:37
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Without the increase of pressure, the velocity of water will not increase. So you see that the water level in front of the rock increases (the pressure increases); the water level in behind of the rock decreases (the pressure decreases). There is a pressure difference, so the velocity of water on both sides of the rock increases. Therefore, without the increase of pressure, there will be no increase of velocity. – enbin Mar 23 '19 at 09:28