Contents

- Q1. Which is lighter (under the same condition) —1 m3 of dry air or 1 m3 of humid air? Mass of one litre of dry air (at S.T.P.) = 1.290 g. Mass of 1 litre of water vapour = 0.308 g.
- Q2. A piece of ice is placed gently on the surface of water in a glass so that when the ice floats, the water comes up to the brim of the glass. What will happen to the level of water when the ice melts? Will it overflow?
- Q3. In the above question, what will happen if instead of water the glass is filled with
- (a) a liquid denser than water.
- (b) a liquid lighter than water
- Q4. What do you mean by thrust?
- Q5. What is pressure? In which unit is it usually expressed?
- Q6. Loaded test-tube placed in pure milk sinks to a certain mark (M). Now some water is mixed with the milk. Will the test tube sink more or less? Explain.
- Q7. (i) Give an application of density.
- (ii) What are its units?
- Q8. Give two practical daily life applications that make use of the concept of pressure.
- Q9. Why are cutting instruments sharpened?
- Q10. What are the factors which affect the pressure at a point in liquid?
- Q11. State the laws of floatation:
- Q12. How is a submarine able to move on as well as go under water?
- Q13. State the factors on which buoyant force depends.
- Q14. Give any three applications of Archimedes’ principle.
- Q15. Why does a camel walk easily on sand but it is difficult for a man to walk on sand though a camel is much heavier than a man?
- Q16. State the laws of liquid pressure.

## Q1. Which is lighter (under the same condition) —1 m^{3} of dry air or 1 m^{3} of humid air? Mass of one litre of dry air (at S.T.P.) = 1.290 g. Mass of 1 litre of water vapour = 0.308 g.

Ans. Mass of 1 litre of dry air = 1.290 g

Mass of 1 litre of water vapour = 0.308 g

Therefore, Density of dry air= \(\frac { 1.290 }{ 1000 } \) = 0.00129 g /cm^{3}

Density of humid air = \(\frac { 0.308 }{ 1000 } \) = 0.000308 g/cm^{3}

Since, the density of dry air is more than that of humid air, the humid air is lighter than dry air.

## Q2. A piece of ice is placed gently on the surface of water in a glass so that when the ice floats, the water comes up to the brim of the glass. What will happen to the level of water when the ice melts? Will it overflow?

Ans. In accordance with the Archimedes’ principle, when the ice melts, the volume of water so formed will be equal to the volume of the displaced water. So, the level of water in the glass will remain unaffected.

## Q3. In the above question, what will happen if instead of water the glass is filled with

## (a) a liquid denser than water.

## (b) a liquid lighter than water

Ans. (a) When the liquid denser than water is filled in the glass, then the volume of water formed by the melting of ice will be more than the volume of the liquid displaced, so the level of water will rise. Therefore, the water will overflow from the glass.

(b) When the liquid lighter than Water is filled in the glass, the level of water in the glass will come down. So, the water will not overflow from the glass.

## Q4. What do you mean by thrust?

Ans. Thrust is the force acting on a body perpendicular to its surface. It has the units of force. The SI unit of thrust is newton (N). Effect of thrust depends upon the area over which it acts.

## Q5. What is pressure? In which unit is it usually expressed?

Ans. Thrust per unit area is called pressure, i.e.,

Pressure = \( \frac { Thrust }{ Area } \)

The SI unit of thrust is newton (N) and the unit of area is metre square (m^{2}). Therefore, unit of pressure is newton per metre square or N/m^{2} or pascal (Pa). As the unit pascal is a very small unit, the pressure is usually expressed in kilopascal unit. i,e., kPa.

## Q6. Loaded test-tube placed in pure milk sinks to a certain mark (M). Now some water is mixed with the milk. Will the test tube sink more or less? Explain.

Ans. When water is added, the density of milk decreases, so the weight of milk-water mixture displaced by immersed part of the tube sink more also decrease and becomes lighter than the weight of loaded tube. So the tube sinks more and will float only when the weight 0f this milk-water mixture displaced by the tube is equal to the weight of loaded tube.

## Q7. (i) Give an application of density.

## (ii) What are its units?

Ans. (i) The density of a given substance (under specified conditions) remains constant and this helps us to determine the purity of the given sample.

(ii) i.e., Relative density = \(\frac { Density\quad of\quad a\quad substance }{ Density\quad of\quad water\quad at\quad 4\quad { \circ }C } \)

As relative density is a ratio of two similar quantities, it has no units.

## Q8. Give two practical daily life applications that make use of the concept of pressure.

Ans. Practical applications based on the concept of pressure:

(i) As we know, the area of a sharp edge is much less than that of a blunt edge. Thus, for the same force, the effective force per unit area, i.e., the pressure is more for the sharp edge than the blunt edge.

(ii) Railway tracks are laid on large sized wooden sleepers, which spread the force due to the weight of the train on a larger area, thus reading the pressure considerably.

## Q9. Why are cutting instruments sharpened?

Ans. Cutting instruments like blades are sharpened so that their area of cross-section decreases and hence pressure exerted by them increases. Therefore, they can easily penetrate a given surface and it becomes easier to cut with them.

## Q10. What are the factors which affect the pressure at a point in liquid?

Ans. Liquid pressure at a point depends on the following three factors:

(i) Depth of point below the free surface

(ii) Density of liquid

(iii) Acceleration due to gravity

The Liquid pressure does not depend on the shape of liquid and area of its base.

## Q11. State the laws of floatation:

Ans. A body floating freely in a fluid, must obey the following laws known as laws of floatation:

(i) A body floats only if its weight is equal to the weight of fluid displaced by its immersed part.

(ii) For the body to float in upright position, the centre of gravity of the floating body and the centre of buoyancy of the fluid displaced by the immersed part of the body must lie on the same straight line.

1st law stated above is necessary for the body to float whereas the 2nd law stated above is necessary for the body to float in upright position.

## Q12. How is a submarine able to move on as well as go under water?

Ans. To move a submarine under water, water is admitted in its special tank, which causes a net increase in weight due to which the submarine goes down in water, in the same manner, to bring the submarine up on the surface of the sea, the water from the tanks is ejected out by compressed air, due to which the net weight of the submarine decreases and it comes up on the surface of the water.

## Q13. State the factors on which buoyant force depends.

Ans. As we know, buoyant force = weight of the fluid displaced by immersed part of body = mg.

But, m= volume of fluid displaced X density of thud = V*e*Therefore, Buoyant force = V

*e*g

Thus, buoyant force depends directly upon:

(i) Volume of the fluid displaced.

(ii) Density of the fluid in which the body is immersed.

(iii) Acceleration due to gravity at the place.

## Q14. Give any three applications of Archimedes’ principle.

Ans. It is used in

(i) designing ships and submarines.

(ii) hydrometers, which are used for determining density of liquids.

(iii) Lactometers, used to determine the purity of a sample of milk.

## Q15. Why does a camel walk easily on sand but it is difficult for a man to walk on sand though a camel is much heavier than a man?

Ans. It is because camel’s feet have large surface area, the force of their weight is distributed over a large area of sand. The pressure produced on sand is small and hence, a camel’s feet do not sink into the sand. On the other hand, a man’s feet have a small surface area, the force of their weight is distributed over a smaller area of sand. The force per unit area or pressure produced on sand is

large in this case due to which a man’s feet sink into sand and it becomes difficult for him to walk on sand.

## Q16. State the laws of liquid pressure.

Ans. The laws of liquid pressure are:

(i) The liquid pressure is same at all points lying on the same horizontal plane in a stationary liquid.

(ii) The liquid pressure at a point inside the liquid is same in all directions.

(iii) The liquid pressure at a point inside the liquid is directly proportional to the depth of that point from the free surface of liquid.

(iv) The pressure of liquid at same depth is different for different liquids. It is directly proportional to density of liquid. That is, for same depth the liquid pressure is larger for liquid of larger density.

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