Solution of Gravitation (Page No - 127 & 128) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 72:
If two equal weights of unequal volumes are balanced in air, what will happen when they are completely dipped in water ? Why ?
Solution :
The two equal weights of unequal volumes which are balanced in air, will get imbalanced when they are completely dipped in water because due to their unequal volumes, they will displace unequal volumes of water and hence suffer unequal loss in weight.

Question 73:
Two different bodies are completely immersed in water and undergo the same loss in weight. Is it necessary that their weights in air should also be the same ? Explain.
Solution :
No, it is not necessary that their weights in air should also be the same. This is because the two bodies have undergone the same loss in weight on completely immersing in water due to their equal volumes and not because of their equal weights, so they may have different weights in air.

Question 74:
A body floats in kerosene of density 0.8 x 10³ kg/m³ up to a certain mark. If the same body is placed in water of density 1.0 x 10³ kg/m³, will it sink more or less ? Give reason for your answer.
Solution :
The body will sink less in water. This is because the density of water is more than that of kerosene due to which water will exert a greater upward buoyant force on the body.

Question 75:
Giving reasons state the reading on a spring balance when it is attached to a floating block of wood which weighs 50 g in air.
Solution :
The reading on spring balance will be zero. This is because the weight of floating block of wood is fully supported by the liquid in which it is floating and hence it does not exert any force on the spring balance.

Question 76:
If a fresh egg is put into a beaker filled with water, it sinks. On dissolving a lot of salt in the water, the egg begins to rise and then floats. Why ?
Solution :
When a lot of salt is dissolved in water, then the density of salt solution becomes much more than pure water. Due to its much higher density, the salt solution exerts a greater upward buoyant force on the egg making it rise and then float.

Lakhmir Singh Physics Class 9 Solutions Page No:128

Question 77:
A beaker full of water is suspended from a spring balance. Will the reading of the balance change :
(a) if a cork is placed in water ?
(b) if a piece of heavy metal is placed in it ?
Give reasons for your answer.
Solution :
The reading of spring balance will not change if a cork is placed in water because cork, being lighter than water, floats in water.
(b)The reading of spring balance will change if a piece of heavy metal is placed in
water because heavy metal being denser than water, sinks in water.

Question 78:
When a golf ball is lowered into a measuring cylinder containing water, the water level rises by 30 cm³ when the ball is completely submerged. If the mass of ball in air is 33 g, find its density.
Solution :
Volume of golf ball = rise in water level = 30 cm³

Question 79:

A boy gets into a floating boat.

• What happens to the boat ?
• What happens to the weight of water displaced ?
• What happens to the buoyant force on the boat ?

Solution :
a) The boat sinks a little more in water, that is, the boat floats lower in water.
b) The weight of water displaced (by the submerged part of the boat) increases.
c) The buoyant force acting on the boat increases.

Question 80:
A y kg sheet of tin sinks in water but if the same sheet is converted into a box or boat, it floats. Why ?
Solution :
The sheet of tin sinks in water because the density of tin is higher than that of water. When the same sheet of tin is converted into a box or a boat, then due to the trapping of lot of ‘light’ air in the box or boat, the average density of the box or boat made of tin sheet becomes lower than that of water and hence it floats in water.