# SSC Worksheet for chapter-5 Why Bodies Float class 9

## Fill in the blanks

1. Force acting on an object perpendicular to the surface is called __________ .
2. Thrust on unit surface area is called _______ .
3. 3. SI unit of pressure is _______ .
4. The pressure at any point in the interior of a liquid is due to _____ of liquid column above that point
5. At sea level, on earth’s surface, atmospheric pressure is about ______ Pa.
6. When the surface area on which a given load is acting decreases, pressure will ______ .
7. SI unit of Thrust is _______ .
8. __________ principle is used in designing ships and submarines
9. Ratio of density of a substance to the density of water is called _________ of the body.
10. ________ is an instrument used to determine density of a liquid
11. ___________ is an instrument used to determine purity of a sample of milk
12. State the factors on which buoyant force due to fluid depends ?
13. Explain the role of buoyant force in determining the floating or sinking of an object in a liquid.
14. State Archimedes’s principle.
15. A ship travelling in the Bay of Bengal enters the river Hooghly. Explain with reasons what will happen to the volume of ship sunk in water in the two cases
16. An ice cube floats in a container holding water. When the ice cube melts, what will happen to level of water in container as compared to the earlier level?
17. Explain why a piece of wood under water rises to the surface of water when released.
18. Why is it easier to pull a bucket full of water while it is in water than when it is out of water ?
19. Describe the functioning of a submarine
20. A man of mass 60 kg stands on sand. Area of contact of his feet with sand is 300 cm2. Find pressure exerted by man on sand (Take g = 9.8 m/s2)
21. Calculate the Relative density of iron having density of 7800 kg/m3

### Solutions

1. Thrust
2. Pressure
3. Pa or N/m2
4. Weight
5. 105
6. Increase
7. Newton
8. Archimedes
9. Relative density or specific gravity
10. Hydrometer
11. Lactometer
12. The buoyant force depends on the volume of liquid displaced and the density of the liquid.
13. Floating and sinking can be explained in terms of the buoyant force. Imagine that you place a block of material underwater and release it. If the upward buoyant force on the block is greater than the downward weight of the block, the block will rise to the surface and float. On the other hand, if the weight of the block is greater than the buoyant force, the block will sink to the bottom.
14. Archimedes’ principle states that, ‘When a body is partially or fully immersed in a fluid it experiences an upward force that is equal to the weight of the fluid displaced by it.’
15. Ship sinks more in river highly than in Bay of Bengal. Reason: density of salt water in sea is more than density of river water. So upward push due to Buoyant force is more in Bay of Bengal.So it will sink less in salt water (Bay of Bengal).
16. Since the ice, when it floats, displaces exactly its weight in water, when it melts, the water it melts into takes the same volume that the ice cube displaced in the water. So the water level should remain the same.
17. Buoyant force acting on wood in water is more than its weight due to less density compared to water. So wood rises to the surface of water when released.
18. When bucket is in water, its weight decreases due to buoyant force acting on it. So it is easy to pull it in water than when it is out of water.
19. A submarine is submerged by allowing water into the main ballast tanks so that its total weight is equal to the weight of the water displaced, hence it attains neutral buoyancy. To bring back the submarine to the surface, the water in the main ballast tanks is blown out by air and the submarine gains positive buoyancy. The main ballast tanks however are either full (when submerged) or empty (when the vessel is on the water surface)
1. M = 60kg, asea = 300ch2

= 300(1/100)2

= 0.03ms2

p = 1

p = force/area

=mg/Area

= 60 x 9.8m/s2 / 0.03m2

p = 19,600 pascals

1. Relative densoty of iron

= denity of iron / density of water at 40c

= 7800 / 1000

= 7.8