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(d) It will spread the force of the bag over the small area of the shoulder of the child
producing less pressure

4. To compare the pressure exerted by a solid iron cuboid while resting on its three
different faces, a student took an iron block of dimensions 15cm x 12cm x 8cm. He
successively places it on the loose sand filled in a tray such that its side of dimensions,
(i) 12cmx8cm (ii) 15cm x 8cm (iii) 15cm x 12cm lie on the sand. On the basis of his
observations he may conclude that the pressure exerted by the iron cuboid is

(a) maximum when it lies on its side of dimensions 12cm x 8 cm

(b) maximum when it lies, on its side of dimensions 15cmx8cm

(d) Same in all the three cases as the thrust is same.

SUBJECTIVE

1. Give the reason: (a) Why does a nail has a pointed tip?
(b) Why does a Knife have sharp edges?

(c) Why do buildings have wide foundations?
2. Why is it easy to walk on sand with flat shoes, than with high heel shoes?

NUMERICALS
1. A force of 150N acts on a surface of area 15cm . Calculate thrust and pressure?
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2. A force of 1200 N acts on the surface of area 10 cm normally. What would be the
thrust and pressure on the surface?

3. The elephant weighs 20,000 N stands on one foot of area 1000 cm . How much
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pressure would it exert on the ground?
4. The pressure of a gas contained in a cylinder with a movable piston is 300 Pa. The
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area of the piston is 0.5 m . Calculate the force that is exerted on the piston.
5.A perpendicular force of 50 N acting on a surface generates a pressure of 250 Pa.

Calculate the area of cross-section of the surface on which pressure is acting.
6.A 3 m height container, sealed, is half filled by fluid. The density of the fluid is ρ = 1.5
Kg/m . What is the pressure at the bottom of the container?
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7.A force of 100N acts on a surface of area 25m ? Calculate thrust and pressure.
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Calculate the changed pressure if the force now is reduced to 25N?

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