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.

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  

 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 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 ∝  Gm1m2 / r2

If distance becomes then the gravitational force becomes four times larger than the previous value.

             r=  r/2

           F=(4Gm1 m2)/r2 =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 ∝  (Gm1 m2)/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. 

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