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A wooden block is floating in a water ta...

A wooden block is floating in a water tank. The block is pressed to its bottom. During the process, work done is equal to :

A

work done against upthrust exerted by the water

B

work done against upthrust plus loss of gravitational potential energy of the block

C

work done against upthrust minus loss of gravitational potential energy of the block

D

None of these

Text Solution

Verified by Experts

The correct Answer is:
C
(c ) Initialy the wooden block floats when partially in mersed in water. Initially, upthrust exerted by water is exactly equal to the weight of the block. But when it is pressed down then more water is displaced. Hecne up thrust exerted by water increases. The force required to press down the block is equal to F = upthrust -mg. Hence, work done by the force F will be equal to work against (upthurst -mg) it means work done by the external force is equal to (work done against upthrust loss of gravitational potential energy of the block).
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Knowledge Check

  • A wooden block is floating on water kept in a beaker. 40% of the block is above the water surface. Now the beaker is kept inside a lift that starts going upward with acceleration equal to g//2 . The block will then

    A
    sink
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    float with 10% above the water surface
    C
    float with 40% above the water surface
    D
    float with 70% above the water surface

  • A wooden block floating in a bucket of water has 4/5 of its volume submerged. When certain amount of an oil is poured into the bucket, it is found that the block is just under the oil surface with half of its volume under water and half in oil. The density of oil relative to that of water is:

    A
    0.7
    B
    0.8
    C
    0.5
    D
    0.6

  • A wooden block is taken to the bottom of a lake of water and then released, it rise up with a

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