RBSE Class 9 Science Chapter 9 Gravitation Solutions

This post provides you a complete solution of Chapter 9 Class 9^th for Rajasthan Board of Secondary Education.

This post contains all the main and important topics which have complete and detailed description.

Board	RBSE
Subject	Science
Chapter	Gravitation
No. of Intext questions	11
No. of End exercise questions	22
Prescribed Book	NCERT Science

 

 

Gravitation Questions- Answers

 

This chapter includes following topics:

 

 

 

1. Gravitation

2. Free Fall

3. Mass

4. Weight

5. Thrust and Pressure

6. Archimedes’ Principle

 

 

Intext Questions

Answers of Questions on Page 102

Q.1 State the universal law of gravitation.

Ans.     

The force of attraction between two particles of the

objects is:

(i)  Directly proportional to the product of the masses of

the objects.

(ii)   Inversely proportional to the square of the distance

between them.

Q.2 Write the formula to find the magnitude of the

gravitational force between the earth and an object on the

surface of the earth.

Ans.      

Answers of Questions on Page 104

Q.1 What do you mean by free fall?

Ans.     

When an object falls towards the earth under the

influence of gravitational force alone, then the object is in

free all.

 

Q.2 What do you mean by acceleration due to gravity?

Ans.     

The acceleration with which an object falls freely

towards the earth is known as acceleration due to gravity.

It is denoted by ‘g’.

Answers of Questions on Page 106

 

Q.1What are the differences between the mass of an object and its weight?

Ans.

 

Q.2 Why is the weight of an object on the moon  its weight on the earth?

Ans.     

Answers of Questions on Page 109

Q1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Ans.

It is difficult to hold a school bag having a thin strap

because the pressure on the shoulders is quite large. This

is because the pressure is inversely proportional to the

surface area on which the force acts. The smaller is the

surface area, the larger will be the pressure on the

surface.

So, in this matter, the contact surface area is very small,

so the pressure exerted on the shoulder is very large.

Q2. What do you mean by buoyancy?

Ans.

The upward force exerted by a liquid on an object

immersed in it is known as buoyancy. All the objects

experience a force of buoyancy when they are immersed

in a fluid. The magnitude of this buoyant force depend

on the density of the fluid.

Q3. Why does an object float or sink when placed on the surface of water?

Ans.

The density of the object and fluids decides the floating

or sinking of the object in water.

i.  If the density of an object is more than the density of

the fluid, then it sinks in the fluid because of the buoyant

force acting on the object is less than the force of gravity.

ii.  If the density of the object is less than the density of

the fluid, then it floats on the surface of the fluid because

the buoyant force acting on the object is greater than the

force of gravity.

Answers of Questions on Page 110

 

Q.1 You find your mass to be 42 kg on a weighing machine. If your mass more or less than 42 kg?

Ans.     

The   machine measures the weight of the body as the

acceleration due to gravity ‘g’ is acting on the body, so the mass reading of 42 kg given by weighing machine is same as the actual  mass of the body. The mass is the quantity of inertia.

   Weighing machine gives the weight of an object.

               Weight = mass ×  g

 

                `Mass = (weight )/g`

  

 

  Thus, mass is less than the weight.

Q2. You have a bag of cotton and an iron bar, each

indicating a mass of 100kg. When measured on a

weighing machine. In reality, one is heavier than other.

Can you say which one is heavier and why?

 

Answer

We have a bag of cotton and an iron bar, both show 100 kg on weighing machine but cotton is heavier.

Reason

 Cotton has a large volume so it displaces more air so upthrust on cotton is more than iron. Due to this upthrust, the machine show less weight than the actual weight.

 So, if both show 100 kg, the cotton must be little heavier t in reality.

 Answers of End Exercise Questions on Page 111 -112

Q.1 How does the force of gravitation between two objects changes when distance between them is reduced to half?

Ans.       

According to the universal law of gravitation –

Gravitational force (F) acting between two objects is

inversely proportional to the square of the distance (r)

between them    

`F ∝  (Gm_1 m_2)/r^2`

If distance becomes then the gravitational force becomes

four times larger than the previous value.

             `r=  r/2`

           `F=(4Gm_1 m_2)/r^2 =4F`

Q.2 Gravitation force acts on all objects in proportion to

their masses. Why, then a heavy objects does not fall

faster than a light object.

Ans.     

The acceleration with which a body falls towards the

earth is constant (=9.8 ms^-2) and independent of the mass

of the body. Thus, all bodies fall with the same

acceleration irrespective of their masses. That is why, a

heavy body does not fall faster than the light body. 

Q.3 What is the magnitude of the gravitational force

between the earth and a 1 kg object on its surface? Mass

of earth is 6 × 10²⁴ kg and radius of earth is 6.4 × 10⁶m.

Ans. 

Q.4 The earth and the moon are attracted to each other

by gravitational force. Does the earth attract the moon

with a force that is greater or smaller or the same as the

force with which the moon attracts the earth? Why?

Ans.     

Gravitational force with which a body A attracts another

body B is equal in magnitude and opposite in direction to

the gravitational force with which a body B attracts the

body A. Thus, the magnitude of force with which the

earth attracts the moon is equal to the magnitude of the

force with which the moon attracts the earth. So, both the

earth and the moon attract each other with equal forces.

Q.5 If the moon attracts the earth, why does the earth not

move towards the moon?

Ans.      

The acceleration produced in the earth due to the force

exerted on it by the moon is very small as the mass of the

earth is very large. That’s why, the movement of the

earth towards the moon is not noticed.

Q.6 What happens to the force between two objects, if:

 (i)  The mass of one object is doubled?

(ii)The distance between the objects is doubled and

tripled?

(iii)         The masses of both the objects are doubled?

Ans.

`F ∝  (Gm_1 m_2)/r^2`

(i)  F is doubled if m₁ and m₂ is doubled because F is

directly proportional to the masses of the objects.

(ii)    F becomes times the original value, if distance (r) is

doubled and F becomes  times the original value of r is

tripled because F is inversely proportional to the square of

the distances between the objects.

(iii)  F becomes four times the original value, if both m₁,

and m₂ are doubled because F is directly proportional to

the product of masses of the objects.

 

Q.7 What is the importance of universal law of gravitation?

Ans.      

i). According to the universal law of gravitation , every object in the universe attracts each other.

ii).All the planets move around the sun due to gravitational force between them.

iii). Moon moves around the earth due to gravitational force between them.

iv).Holding of atmosphere near the earth.

v).Tides in the sea occur due to gravitational force.

Q.8 What is the acceleration of free fall?

Ans.      

When objects fall towards the earth under the effect of gravitational force alone, then they are said to be in free fall.

Acceleration of free fall = 9.8 ms-² 10 ms^-2.

Q.9 What do we call the gravitational force between the earth and an object?

Ans.      

Force of gravity or gravitational acceleration. It is denoted by ‘g’.

 

Q.10Amit buys few grams of gold at the poles as per the

instruction of one of his friends. He hands over the same

when he meets him at the equator. Will the friend agree

with weight of gold bought? If not, why?

Ans.            Weight = mg

Since we know that value of g is greater at the poles than

at the equator, so the weight of gold at the poles will be

greater than the weight of gold at the equator. Hence, his

friend will say that the weight of gold is less than as told

by Amit. 

Q.11  Why will a sheet of paper fall slower than one that

is crumpled into a ball?

Ans.      

Since the area of a sheet of paper is more than the area of

the paper crumpled into a small, therefore, a sheet of

paper falls slower than one that is crumpled into a ball.

 

Q.12 Gravitational force on the surface of the moon is

only as strong as gravitational force on the earth. What is

the weight in newtons of a 10 kg object on the moon and

on the earth?  

Ans.

Q.13 A ball is thrown vertically upwards with a velocity

of 49 m/s. Calculate: (i) the maximum height to which it

rises (ii) the total time it takes to return to the surface of

the earth.

Ans

Q.14  A stone is released from the top of a tower of

height 19.6m. Calculate its final velocity just before

touching the ground.

Ans. 

(-ve sign is retained as per sign convention)

Q.15 A stone is thrown vertically upward with an initial

velocity of 40 m/s. Taking g = 10 ms^-2, find the

maximum height reached by the stone. What is the net

displacement and the total distance covered by the stone?

Ans. 

Q.16Calculate the force of gravitation between the Earth

and the sun that the M_e = 6 × 10²⁴ kg , M_s = 2 × 10³⁰,

kg.  The average distance between the two is 1.5 × 10¹¹

m.

Ans. 

Q.17     A stone is allowed to fall from the top of a tower

100 m height and at the same time another stone is

projected vertically upwards from the ground with a

velocity of 25 m/s. Calculate and when where the two

stones will meet?

Ans.

Q.18 A ball thrown up vertically returns to the thrower

after 6 s.

 Find:   

(a) velocity with which it was thrown up.

 (b) The maximum height it reaches.

 (c)  Its position after 4 s

Ans.

Q19. In what directions does the buoyant force on an

object immersed in a liquid act?

Ans.  

An object immersed in a liquid experiences buoyant force

in the upward direction.

Q20. Why does a block of plastic released under water come up to the surface of water?

Ans. 

When an object is immersed in water, two forces act on it. One is the gravitational force which pulls the object downwards and the other is buoyant force which pushes the object upwards.

If the upward buoyant force is greater than the downward force then the object comes up to the surface of the water , sue to this reason a block of plastic released under water comes up to the surface of water.

Q21. The volume of 50g of a substance is 20 cm³. If the density of water is 1g cm^-3 , will the substance float or sink?

Ans. 

Q22. The volume of a 500 g sealed packet is 350cm³. Will the packet float or sink in water if the density of water is 1g cm^-3? What will be the mass of the water displaced by this packet?

Ans.