Chapter 10 Gravitation

Rajasthan Board RBSE Class 9 Science Chapter 10 Gravitation

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Rajasthan Board RBSE Class 9 Science Chapter 10 Gravitation

Rajasthan Board RBSE Class 9 Science Chapter 10 Gravitation

Gravitation Textbook Questions Solved

Objective Type Questions

Question 1.
Newton’s law of gravitation is called universal law of gravitation because:
(A) it is always valid for forces of attraction
(B) it is applicable to all particles and members of the solar family
(C) it is applicable for all masses at all distances, independent of the medium
(D) None of these
Answer: C

Question 2.

The necessary force required to circulate a body in circular motion is:
(A) gravitational force
(B) frictional force
(C) centripetal force
(D) none of the above
Answer: C

Question 3.
Universal gravitational constant, G depends:
(A) on the nature of the particle
(B) on the medium present between the particles
(C) on time
(D) does not depend on any of these factors.
Answer: D

Question 4.
If the weight of a man on earth’s surface is 60 N, then what will be the weight on the surface of the moon?
(A) 60 N
(B) 10 N
(C) 20 N
(D) 0 N
Answer: D

Question 5.
A body of mass m is shifted on the surface of a deep mine, then:
(A) its mass increases
(B) its mass decreases
(C) its weight decreases
(D) its weight increases
Answer: C

Question 6.
we double the distance between two masses, the gravitational force between the masses:
(A) remains unchanged
(B) becomes one fourth
(C) becomes half
(D) becomes twice
Answer: C

Rajasthan Board RBSE Class 9 Science Chapter 10 Gravitation

Gravitation Very Short Answer Type Questions

Question 7.
From where do satellites receive the necessary centripetal force to revolve around a planet?
Answer
The necessary centripetal force required is received by the gravitational force, imposed by the planet on satellites.

Question 8.
How does the gravitational mass of a body is determined in an artificial satellite?
Answer
The gravitational mass of a body cannot be determined, because the artificial satellite is like a free-falling body.

Question 9.
What changes will arise in force of gravitation between the masses, if the distance between them is doubled?
When d is doubled, the gravitational force will become one fourth.

Question 10.
Find the weight of an object on the surface of the earth, if its mass is 10 kg.
Answer
We are given that
m = 10 kg ; g = 9.8 m/s2; weight (w) = mg w = 10 x 9.8 w = 98 kg m/s2 = 98 N

Question 11.
Write the formula of the gravitational force between two bodies.
Answer
Let the mass of two interacting bodies be m1 and m2 and d be the distance between
them, then the force of gravitation/gravitational force is given by:

where G is called gravitational constant.

Question 12.
Name the principle on which ball pen works.
Answer
It works because of the force of gravity. It works on the principle of Universal Law of Gravitation.

Question 13.
A man can jump higher on the moon. What is the cause of it?
Answer
On the moon, the acceleration due to gravity is  th of that on earth. So, a man can jump higher on the surface of the moon.

Question 14.
Two bodies of masses 1 kg each are situated 1 metre apart. Find the gravitational force between them.
Answer
We are given that: mx = m2 = 1 kg ; d = 1 m

Question 15.
Earth is continuously pulling moon towards its centre by gravitational force, then why does not moon fall on the earth?
Answer
The gravitational pull of earth provides the necessary centripetal force to moon for its orbital motion around the earth, due to which the moon does not fall on the earth.

Question 16.
What is the cause of ocean tides?
Answer
The earth and the moon pull each, other due to the gravitational force of attraction. Water on the earth (ocean) is attracted by both the earth and the moon. The molecules in water are not rigidly fixed. Therefore, when the moon and the sun both lie on the same side of the earth (new moon) or on the opposite side of the earth (full moon) with the ocean, the combined gravitational pull causes a bulge in the ocean water. This bulging out of the ocean water in the direction of the moon is called tide.

Gravitation Short Answer Type Questions

Question 17.
State Newton’s law of gravitation.
Answer
“Every particle in the universe attracts every other particle with a force, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them”.

Question 18.
At which place on the surface of the earth, the acceleration due to gravity is more and why?
Answer
The value of g varies from point to point in the universe, according to the values of M and r. Since the earth is flattened at poles and V reduces. Now since
.
Therefore, the value of g is more at poles of the earth than the equator.

Question 19.
What is the value of universal gravitational constant? Write also its S.I. unit.
Answer
The value of G universal gravitational constant is 6.67 x 10-11 Nm2/kg2It’s S.I. the unit is Nm2/Kg2

Question 20.
What is acceleration due to gravity? Write its formula.
Answer
The acceleration produced in a body due to the gravitational pull of the earth, near its surface is called acceleration due to gravity. It is denoted by ‘g’. i.e.,

Here, Me= mass of earth, Re = radius of earth and G is universal gravitational constant.

Question 21.
What are the differences between the mass of an object and its weight?
Answer

Mass	Weight
1. It is the amount of matter in a body.	1. It is the force with which the earth pulls of a body.
2. It is a constant quantity, remains same at all the places in-universe.	2. It is the variable quantity and changes with the change in the value of acceleration due to gravity.
3. It is measured by a physical balance.	3. It is measured by a spring balance.
4. It’s S.I. the unit is a kilogram.	4. It’s S.I. the unit is Newton.

Question 22.
What do you mean by free fall? Give examples.
Answer
Whenever objects fall towards the earth only under the gravitational force of the earth (with no other forces acting on it) then we say that the objects are in the state of free fall.
Example: An object dropped from a height like a stone on the ground.

Question 23.
A man moves from earth poles to equatorial line, then what changes will be there in man’s weight and why?
Answer
When a man moves from earth’s pole to equatorial line, the weight of man will decrease, because on poles the value of acceleration due to gravity is more as compared to the equatorial line.

Question 24.
What is weightlessness? Give two examples.
Answer
Weightlessness is a situation in which the effective weight of the body becomes zero.
Example 01: When a body is lying in a freely falling lift, then acceleration, a = g. So, the effective acceleration due to gravity in the left
g’ = g – a⇒g’=g-g = 0
So, the weight of the body is equal to zero.
Example 02: When the body is inside a satellite, which is orbiting around the earth; the gravitational pull on the body due to earth is balanced by the centrifugal force on the body, due to which the effective weight of the body becomes zero.

Question 25.
Write the equations of motion for a freely falling body. Also, write the meaning of symbols used in the equations.
Answer
Equations of motion of an object under free fall :The initial velocity of the object remains zero when the object is under free fall. In this situation, the three equations of motion i.e. v = u + gt
s= ut + gt
v2 = u2 + gt2
v2 = u2 + 2gh will become,
ν=gt
s = gt2 and v2 = 2gh
Here, u = final velocity, t = time, g = acceleration due to gravity, h = height attained by the body.

Question 26.
A body is thrown upwards with velocity ‘u ’ and it attains a height h. Write the equation of motion for the body.
Answer
When an object is thrown upwards, the value of acceleration due to gravity (g) is negative.
Thus, equations of motion are:
(1) v = u- gt
(2) h = ut-  gt2(3) v2 = u2 – 2gh

Question 27.
What problems occur for astronaut during weightlessness?
Answer
(a) In weightlessness state, the major problem faced by astronauts in the spacecraft is eating, drinking water, etc. It occurs because these eatable materials do not stick with plate properly and glass water does not pour.
(b) The space flight for a long time adversely affects the astronauts.
(c) Inside the aircraft, all the objects are in a floating position.

Question 28.
Before Kepler and Newton, name some ancient astrologers. Also, write their achievement in the gravitational field.
Answer
(a) In the fifth century, Indian astrologer Aryabhatta gave the geocentric model to understand the motion of the planet.
(b) After Aryabhatta, Bhashkaracharya gave his theory about gravity and planetary motion in his book ‘SIDDHARTH SHIROMANI’. Bhaskaracharya estimated the radius of the earth and its circumference.

Question 29.
Why the state of weightlessness occurs in artificial satellites and not in natural satellites? Explain.
Answer
On natural satellite like the moon, the man does not experience weightlessness because the mass of moon (natural satellite) is more and it applies a force of gravity on man. But in artificial satellite, the mass of satellite is less, so it imposes less gravitational force on the object due to which the astronaut experience the weightlessness.

Rajasthan Board RBSE Class 9 Science Chapter 10 Gravitation

Gravitation Long Answer Type Questions

Question 30.
Two spheres each of mass 10 kg are 50 cm apart. What is the gravitational force between them?
Answer
We are given that: m1 = m2 = 10 kg

Question 31.
Find the gravitational force on a body of man 40 kg situated on the surface of the earth;
if radius of earth (R) = 6400 km and mass of earth (M) = 6 x 1024 kg.
Answer
We are given that,
Mass of a body (m) = 40 kg Mass of earth (M) = 6 x 1024 kg
Radius of earth (R) = 6400 km = 64 00 x 103 = 6.4 x 106 m
Gravitational constant (G) = 6.67 x 10-11  Nm2/kg2
According to Newton’s gravitational law,

Question 32.
A stone is thrown vertically upwards with a speed of 20m/s. If g = 10m/s2, find
(1) Time is taken by the stone to reach the maximum height.
(2) Total distance covered by the stone to reach the maximum height.
Answer
We are given that: Initial velocity (u) = 20 m/s
When an object attain maximum height, then final velocity (ν) = 0
a = g = 10m/s2, time (t) = ?
(1) From first equation of motion, we have v = u – gt
⇒ 0 = 20-10 x t ⇒ 10t = 20 ⇒ t = 2 sec

(2)  From third equation of motion, we have
v2= u2 – 2gh
⇒ (0)2 = (20)2 -2 x 10 x h
⇒ 20h = 20 x 20
⇒ h = 20 m

Question 33.
If the radius and mass of moon are 1738 km and 0.073 x 1024 kg, respectively. Find the acceleration due to gravity on the surface of the moon.
Answer
We are given that: Radius of moon (Rm) = 1738 km
Mass of moon: M = 0.073 x 1024 kg
Acceleration due to gravity (gm) = ?
According to formula

Question 34.
A stone is released from the top of a tower 125 m high. Then find:
(1) the time with which the stone strikes the ground.
(2) the final velocity of the stone. (Take g = 10 m/s2)
Answer
We are given that: Height of tower (h) = 125 m
Initial velocity (u) = 0
g = 10 m/s2(1) From 2nd equation of motion
For downward motion: h = ut + gt2⇒ 125 = 0 x t +  x 10 x t2 =125 = 5t2 ⇒ t2 = 25 ⇒ t = 5sec 2

(2) From 1st equation of motion,
v = u + gt
⇒ u = 0 + 10 x 5
⇒ v = 50 m/s

Question 35.
If the diameter of the earth becomes half of its original value, then mass becomes 1/8 of its original value. Find the acceleration due to gravity of this part.
Answer
We are given that: Diameter of the earth becomes half
Let R be the new radius of earth and r be the original radius.

Hence, the acceleration due to gravity becomes half of its original value

Gravitation Addition Questions Solved

Multiple Choice Questions (MCQs)

Question 1.
A ball is thrown vertically upwards. The acceleration due to gravity:
(A) is in the upward direction
(B) is in the downward direction
(C) is in the horizontal direction
(D) always in the direction of motion
Answer: B

Question 2.
The value of acceleration due to gravity:
(A) is least at the equator of the earth
(B) is least at the poles of the earth
(C) is same everywhere on the earth
(D) increases from poles to the equator
Answer: A

Question 3.
The gravitational force between two objects is F. If the masses of both the objects are halved, keeping the distance between them the same, then the gravitational force would become:
(A) 1/2 F
(B) 1/4 F
(C) 2 F
(D) 4 F
Answer: B

Question 4.
Two bodies of different masses are falling freely near the surface of the earth. They:
(A) have the same velocity at any instant
(B) move with different accelerations.
(C) experience forces of the same magnitude
(D) undergo a change in their inertia.
Answer: A

Question 5.
The law of gravitation is true for:
(A) the earth and any object only
(B) earth and the sun only
(C) any two massive bodies
(D) two charged bodies only
Answer: C

Question 6.
At the centre of the earth:
(A) the mass of a body is zero but the weight is not zero
(B) the mass of a body is not zero but the weight is zero
(C) both mass and weight of a body are zero
(D) both mass and weight of a body are not zero
Answer: B

Question 7.
The gravitational force of attraction between two objects placed at distance R is proportional to:
(A) R
(B) R2(C) 1/R
(D) 1/R2Answer: D

Question 8.
If the distance between two objects is doubled, the gravitational force between them:
(A) remains unchanged
(B) becomes half
(C) becomes one-fourth
(D) becomes double
Answer: C

Question 9.
The S.I. unit of weight is:
(A) kg
(B)kg-1 nr1(C) N-m
(D) N
Answer: D

Question 10.
A stone is dropped from the top of a tower 40 metre high. Its velocity after it has fallen for 2s is:
(Take g = 10m/s2).
(A) – 10 m/s
(B) 10 m/s
(C) – 20 m/s
(D) 20 m/s
Answer: C

Rajasthan Board RBSE Class 9 Science Chapter 10 Gravitation

Gravitation Very Short Answer Type Questions

Question 1.
What will be the weight of an object on the earth whose mass is 10 kg? [g = 10 m/s2]
Answer
w = mg
(Here m = 10 kg,
g = 10 m/s2)
w = 10 x 10 kg m/s2w = 100 N

Question 2.
How Newton’s second law of motion, related to the universal law of gravitation?
Answer
By Newton’s second law of motion

Question 3.
What will be the weight of the book whose mass is 500 gm, placed at c height equal to the radius of the earth?
Answer
When height is equal to the radius of the earth i.e. h = R

Question 4.
The gravitational force acting on all objects is proportional to their masses. Why, then, a heavy object does not fall faster than a light object?
Answer
The acceleration with which a body falls towards earth is constant, i.e., 9.8m/ s2 and independent of the mass of the body. Thus, all the bodies fall with the same acceleration irrespective of their masses. That is why a heavy body does not fall faster than a light object.

Question 5.
What is the source of the centripetal force that a planet requires to revolve around the Sun? On what factors does the force depend?
Answer
The centripetal force is provided by the gravitational pull of the Sun on the planet.
The centripetal force depends on-

1. Mass of the Sun
2. Mass of the planet
3. The distance between the Sun and the planet.

Question 6.
What is the difference between G and g?
OrDistinguish between the force of gravity and gravitational force.
Answer
G (Gravitational force)

1. Any two objects in the universe attract each other by a force called gravitational force.
2. It produces almost negligible acceleration.
3. It is not the measure of weight, g (gravity)
4. The force of gravitation exerted by the earth is called gravity.
5. Gravity causes the object to accelerate. It measures the weight, i.e., w = mg.

Question7.
How does the force of gravitation between two objects change when the distance between them is reduced to half?
Answer
Force of gravitation is inversely proportional to the square of the distance between the two objects if all other factors remain the same, i.e.,

Thus, the force of gravitation increases by 4 times.

Question 8.
If the moon attracts the earth, why does the earth not move towards the moon?
Answer
The force exerted by the earth and the moon on each other is insufficient to move the earth, on account of its huge mass.

Question 9.
What is the mass and weight of an object on the surface of the moon, if on the earth its mass is 12 kg?
Answer
Mass of the object on earth is 12 kg. It will be 12 kg on the moon also. Value of ‘g’ on the moon is one-sixth of its value on the earth.
Weight of the object on earth = mg = 12 x 9.8 = 117.6
N Weight of the object on moon 1
= 12 x  x 9.8 = 19.6 N.

Question 10.
A ball thrown upwards reaches a certain height and then falls back to the earth. Why?          ,
Answer
The ball thrown upwards from the earth goes up against the force of gravity. So, its velocity goes on decreasing at the rate of 9.8 m/s2. It becomes zero at a certain height. So, the ball stops there for a moment and then, begins to fall back to the earth due to the force of gravity, as any object dropped from that height.

Gravitation Short Answer Type Questions

Question 1.
How does the force of gravitation between two objects changes, when the distance between them is reduced to half?
Answer
Force of gravitation between the two objects is given by

Thus, the force of gravitation increases by 4 times.

Question 2.
What is the importance of the universal law of gravitation?
Answer
The universal law of gravitation successfully explained the planetary motion in our solar system. The gravitational force is responsible for so many natural phenomenon like the formation of tides in seawater, the existence of the atmosphere near the earth and many more.

Question 3.
Gravitation force on the surface of the moon is only 1/6 as strong as gravitational force on the earth. What is the weight in Newtons of 10 kg object on the moon and on the earth?
Answer
Mass of the object (m) = 10 kg
Acceleration due to gravity on earth = g Acceleration due to gravity on moon = g
Now,

Weight of an object on earth We = mge = 10 x 9.8 = 98 Newton
Weight of an object on the moon
Wm = mgm =10×1.635 = 16.35 Newton

Question 4.
What happens to the force between two bodies, if the distance between them is tripled and the masses of the objects is doubled?
Answer
The force between the objects is given

Gravitation Long Answer Type Questions

Question 1.
(a) State Newton’s law of gravitation. Obtain an expression for the gravitational force between two mass bodies.
(b) What is the value of G? Also, write its S.I. unit.
Answer
(a) According to this law, the force of gravitation acting between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The direction of the force is along, the line joining the two masses.

where G is proportionality constant, called universal gravitational constant.
(b) Value of G is 6.67 X 10_11 N-m2/kg2

Question 2.
(a) What do you understand by free fall? Derive an expression for acceleration due to gravity.
(b) Mass of earth is 81 times that of the moon, and its diameter is 3.6 times that of the moon. Compare the acceleration due to gravity on the earth and on the moon.
Answer
(a) When an object moves towards earth vertically under the effect of gravitational force only, its motion is said to be free fall.
Acceleration due to gravity-
The gravitational force between an object and earth is given by

On putting values of mE and RE, the value of g at the surface of earth obtained is g = 9.8 m/sec.
(b) Let the mass of earth = m, Radius of earth = Rm
mass of the Moon = Rm
A radius of Moon = Rm
If acceleration due to earth is g and due to the moon is gm then

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