The force required by a man to move his limbs immersed in water is smaller than the force for the same movement in air. Why? 

Upward thrust acting on the limbs is more in water than in air.
Therefore, the force required to move the limbs in water is smaller than that in air.
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Ships are made up of metals whose densities are more than density of water. Even then the ships float in water. Explain why.

The ships float in water even though they are made up of metals whose densities are more than the density of water. The ships are made to float in water by making these metals hollow. By making the metals hollow, the effective density of the ship becomes less than that of water. 
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An ice piece floats in a glass of water. How does the level change when whole of ice melts?

When the whole ice melts, the water level will reamin unchanged. The floating ice piece will displace the volume of water equal to the weight of ice. When the ice melts the volume of water so formed is exactly equal to the volume of water displaced by ice. Hence, the water level will remain unchanged. 
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Why does the ship rise as it enters the sea from river?


The density of sea water is greater than the river water. Therefore, to balance the weight of ship, the volume of sea water displaced by the ship will be less than that of the river water. Hence on entering the sea from river the ship rises a little. 
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A boat floating in a water tank is carrying a number of large stones. If the stones are unloaded into water, what will happen to the water level?

The level of water will fall when stones are unloaded into water. The density of stones is more than the density of water. So, when the stones are dropped into the water, the stone replaces the volume of water equal to the volume of stones. 
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