Solved Examples

1.    Water is filled in a flask upto a height of 20 cm. The bottom of the flask is circular with radius 10 cm. If the atmospheric pressure is 1.01 × 105 Pa, find the force exerted by the water on the bottom. Take g = 10 m/s2 and density of water = 1000 kg/m3.            


2.    A 700 g solid cube having an edge of length 10 cm floats in water. How much volume of the cube is outside the water ? Density of water = 1000 kg/m3.    

3.    Figure shows a liquid being pushed out of a tube by pressing a piston. The area of cross-section of the piston is 1.0 cm2 and that of the tube at the outlet is 20 mm2. If the piston is pushed at a speed of 2 cm/s, what is the speed of the outgoing liquid ?

4.    Figure shows a liquid of density 1200 kg/m3 flowing steadily in a tube of varying cross-section. The cross-section at a point A is 1.0 cm2 and that at B is 20 mm2, the points A and B are in the same horizontal plane. The speed of the liquid at A is 10 cm/s. Calculate the difference in pressures at A and B. 
5.    A water tank is constructed on the top of a building. With what speed will the water come out of a tap 6.0 m below the water level in the tank ? Assume steay flow and that the pressure above the water level is equal to the atmospheric pressure.

Questions for Short answer
1.    It is always true that the molecules of a dense liquid are heavier than the molecules of a lighter liquid ?

2.    If someone presses a pointed needle against your skin, you are hurt. But if someone presses a rod against your skin with the same force, you easily tolerate. Explain.

5.    The free surface of a liquid resting in an inertial frame is horizontal. Does the normal to the free surface pass through the

centre of the earth ? Think separately if the liquid is (a) at the equator (b) at a pole (c) some where else.

6.    A barometer tube reads 76 cm of mercury. If the tube is gradually inclined keeping the open end immersed in the mercury reservoir, will the length of mercury column be 76 cm, more than 76 cm or less than 76 cm?

7.    A one meter long glass tube is open at both ends. One end of the tube is dipped into a mercury cup, the tube is kept vertical and the air is pumped out of the tube by connecting the upper end to a suction pump. Can mercury be pulled up into the pump by this process?

8.    A satellite revolves round the earth. Air pressure inside the satellite is maintained at 76 cm of mercury. What will be the height of mercury column in a barometer tube 1 m long placed in the satellite ?

9.    Consider the barometer shown in figure. If a small hole is made at a point P in the barometer tube, will the mercury come out from this hole ?
10.    Is Archimedes’