Class 9 Science Chapter 10 Gravitation Important questions
Here is some important questions of chapter 10 gravitation class 9. One word questions, very short questions and short question with detailed answers.
Class 9 Gravitation Questions Science
Chapter – 10 Gravitation
One word Questions –
Q- 1 The universal law of gravitation was given by –
Ans – Newton
Q- 2 The tidal waves in the sea are primarily due to –
Ans – Gravitation effect of moon on earth
Q- 3 If the radius of earth were to shrink by 1%, its mass remaining the same, the acceleration due to gravity on earth’s surface would –
Ans – Increase ( R decreases, g increases g = GM/R² )
Q- 4 In the relation F = GMm/R² , the quantity G is –
Ans – Universal constant of nature
Q- 5 10 kg weight means –
Ans – 98 N ( W = mg = 10 x 9.8 = 98 N)
Q- 6 The value of quantity G in the law of gravitation –
Ans – is independent of mass and radius of earth
Q- 7 The value of g is maximum at –
Ans – Poles of the earth.
Q- 8 The weight of an object at the centre of the earth of radius R is –
Ans – Zero
Q- 9 The SI unit of g is –
Ans – m/s²
Q- 10 The SI unit of G is –
Ans – N kg²/m²
Q- 11 An object weighs 10 N when measured on the surface of earth. What should be its weight on moon ?
Ans – 1.67 N ( Weight of an object on the moon is 1/6th of weight of an object on the earth so 10/6 = 1.67 N)
Q- 12 ‘G’ stands for –
Ans – Universal Gravitational Constant
Q- 13 ‘g’ stands for –
Ans – Acceleration due to gravity
Q- 14 When a body is dropped from a height, what is its initial velocity?
Ans – Zero
Q- 15 When a body is thrown vertically upwards, what is its final velocity?
Ans – Zero
Q- 16 Is weight a force –
Ans – Yes ( F = W = mg)
Q- 17 If the mass of a body is 9.8 kg on the earth, what would be its mass on the moon ?
Ans – 9.8 kg ( It will be remain same on the moon as mass of any object will be constant everywhere)
Q- 18 At what place on the earth’s surface is the weight of the body minimum?
Ans – At the equator
Q- 19 Value of G –
Ans – 6.67 x Nm²/kg²
Q- 20 Value of g –
Ans – 9.8 m/s²
Gravitation Class 9 Very Short questions –
Q- 1 Why is ‘G’ called the universal gravitational constant –
Ans – The constant ‘G’ is universal because it is independent of the nature and sizes of bodies, the space where they are kept and at the time at which the force is considered.
Q- 2 The earth is acted upon by gravitation of sun, even though it does not fall into the Sun. Why?
Ans – The gravitational force is responsible for providing the necessary centripetal force which binds the earth to move around the Sun at the its orbit or a defined path. So earth does not fall into the Sun.
Q- 3 What keeps the moon in uniform circular motion around the earth?
Ans – Gravitation force which is responsible for centripetal force which binds moon to keep around the earth in uniform circular motion.
Q- 4 Why can one jump higher on the surface of the moon than on the earth?
Ans – The value of acceleration due to gravity(g) on the moon’s surface is nearly 1/6th to that of the surface of the earth.
Q- 5 How is gravitation different from the gravity?
Ans – Gravitation is the force of attraction between any two bodies while gravity is the acceleration due to gravitational force.
Q- 6 If the distance between the objects is doubled, what happens to the force between two objects ?
Ans – The gravitational force will be 1/4th of the original value,
F = GMm/d²
If d = 2d
F’ = GMm/(2d)²
F’ = F/4
Q- 7 If the mass of the both objects is doubled, what happens to the force between two objects ?
Ans – It will be four times of the original value.
F = GMm/d²
F’ = G 2M x 2m/d²
F’ = 4GMm/d²
F’ = 4F
Q- 8 SI unit of mass and weight –
Ans – SI unit of mass is kilogram, SI unit of weight is Newton.
Q- 9 What do we call the gravitational force between the earth and an object ?
Ans – Force of gravity or gravity.
Q- 10 Newton’s law of gravitation is applicable to –
Ans – All bodies in the universe
Q- 11 A body falling under gravity moves with a constant-
Ans – Acceleration
Q- 12 An apple falls from a tree because of gravitational attraction between the earth and apple. If F1 is the magnitude of force exerted by the earth on the apple and F2 is the magnitude of force exerted by apple on the earth, then –
Ans – F1 and F2 both are equal
Q- 13 Why does formation of tides takes place in sea or ocean?
Ans – The tides in the sea formed by the rising and falling of water level are due to the gravitational force of attraction which the sun and moon exert on the water surface in the sea.
Q- 14 What does a small value of G indicate –
Ans – A small value of G indicates that the force of gravitational attraction between two ordinary sized objects is a very weak force.
Gravitation Class 10 Short Question Answers –
Q- 1 Mention any four phenomena that the universal law of gravitation was able to explain.
Ans – The universal law of gravitation –
- The force binds us to the earth
- The motion of the moon around the earth.
- The motion of planets around the sun.
- The tides due to the moon and the sun.
Q- 2 (a) When does an object show weightlessness?
(b) If the small and big stones are dropped from the roof of a house simultaneously, they will reach the ground at the same time. Why?
Ans- (a) When an object is in the state of free fall under the effect of gravity. Weightlessness is a state when an object does not weigh anything.
(b) The acceleration due to gravity does not depend upon the mass of the object. Both the object fall with the same acceleration g = 9.8 m/s². So a big stone will fall with the same acceleration as a small stone. So both the stones will reach the ground at the same time when dropped simultaneously.
Q- 3 (a) What is the source of centripetal force that a planet requires to revolve around the sun? On what factors does that force depend?
(b) Suppose gravity of earth suddenly becomes zero, then which direction will the moon begin to move if no other celestial body affects it?
Ans – (a) That source is Gravitational force. Gravitational force depends on the product of the masses of the planet and sun and the distance between them.
(b) The moon will move in a straight line in the direction of it was moving when it was experiencing centripetal force provided by the gravitational force of the earth.
Q- 4 On the earth, a stone is thrown from a height in a direction parallel to the earth’s surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why?
Ans – Both stone will take same time to reach the ground because the two stones fall from the same height.
For both the stones
Initial velocity, u = 0
s = ut + ½ gt²
= 0 + ½ gt²
t² = 2s/g
t = √2s/g
So time is same for equal heights.
Q- 5 Prove that if a body is thrown vertically upward, the time of ascent is equal to the time of descent.
Ans – Upward motion –
v = u – gt1
in upward motion – v = 0
0 = u – gt1
t1 = u/g
For downward motion –
v = u + gt2
in downward motion – u = 0
v = 0 + gt2
as the body falls back to the earth with the same velocity it was thrown vertically upwards.
v = u
t2 = v/g
t2 = u/g
So t1 = t2
Q- 6 Why does a body reach the ground quicker at poles than at the equator when dropped from the same height?
Ans – The acceleration due to gravity is more at the poles than at the equator. The time taken for a body is less if the acceleration due to gravity is more when the initial velocities and the distance travelled are the same. So, with same height also body reaches the ground quicker at poles than at the equator.
Q- 7 When the radius of the earth becomes twice of its original radius without any change in its mass. Then what will happen to weight?
Ans – W = GMm/r²
W1 = GMm/(2r)²
W1 = GMm/4r²
W1 = W/4
Weight will be one fourth of the original.
Q- 8 Two objects of masses m1 and m2 having the same size are simultaneously from heights h1 and h2, respectively. Find out the ratio of time they would take in reaching the ground. Will this ratio remain the same if (i) one of the objects is hollow and the other one is solid, (ii) both of them are hollow,size remaining the same in each case? Give reasons.
Ans – s = ut + 1/2gt1² u = 0, s = h
h1 = 1/2gt1²
h2 = 1/2gt2²
t1/t2 = √h1/h2
Ratio will not change in either case because acceleration remains the same. In case of free fall acceleration does not depend upon mass and time.
Differentiate questions –
Q- 1 Acceleration due to gravity(g) and Universal gravitational constant(G)
Acceleration due to gravity(g)
Universal gravitational constant(G)
2 g is a vector quantity.
2 G is a scalar quantity.
3 It is different at different places on the earth’s surface.
3 The value of ‘G’ is universally constant. Its value is same at any place.
G = 6.67 x 10^-11
Q – 2 Mass and Weight
2 It is a scalar quantity.
2 It is a vector quantity.
3 SI unit of mass kilogram(kg).
3 SI unit is newton (N)
4 It will be same everywhere.
4 It depends on the gravitation on that place.
Gravitation class 9 Some Long Questions –
Q- 1 Derive expression for force of attraction between two bodies and derive the universal gravitational constant.
Q- 2 Define acceleration due to gravity. Derive an expression for acceleration due to gravity in terms of mass of the earth (M) and universal gravitational constant(G).
Q- 3 Show that the weight of an object on the moon is 1/6th of its weight on the earth.