A cork ball is tied to one end of a string and the other end of the string is tied to the bottom of a container filled with water as shown in figure. When the string is gently cut. The ball rises to position B where its velocity is V. Then

Moderate

A

As kinetic and gravitational potential energy of the ball both increase, it violates the law of conservation of energy.
B

Kinetic energy of the ball increases but its gravitational potential energy decreases.
C

Gravitational force is doing positive work on the ball.
D

kinetic and gravitational potential energy of the ball both increase, but the law of conservation of energy is not violated.

Solution

As the ball rises from position A to position B, the vacuum created by the ball at position A is filled up by water which previously filled the volume at position B. So gravitational potential energy of this much water decreases. A part of thus lost gravitational potential energy is used by the ball to increase its kinetic energy and the balance amount of lost energy is used to increase the potential energy of the ball.
work done by the force due to pressure difference is equal to SUMOF increase in k.E and INCREASE IN POTENTIAL ENERGY

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