Bernoulli's theorem and dancing ball
By The pressure inside the moving fluid is less in comparison to the pressure inside the static fluid. This is quantitatively described by the Bernoulli's equation. This demonstration is one of the common manifestations of Bernoulli's theorem.
Procedure
Take a tennis ball and fix one end of a string at one point of the ball using cellophane tape. Let the length of the string be about 10 cm. Open the tap in basin or sink to have water coming out as stream. Hold the end of the string to suspend the ball vertically. Move your hand towards the water stream so that a portion of the ball goes in the water stream. Don't put the whole ball in the stream. Water falls on one side of the ball and not the other side. Very slowly move your hand away from the tap. The ball will not come with you. The string will become inclined. See the largest inclination you can give to the string.
Discussion
The experiment shows that there is horizontal force on the ball towards the water stream which balances the horizontal component of the tension in the string. From where does this horizontal force come? The pressure \(P_1\) in the water stream is smaller then \(P_0\) due to the air away from the stream. This is what Bernoulli's equation describes. The air on the farther side of the ball exerts larger horizontal force than what the water exerts. Thus, stream appears to attract the ball.
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