STD 9 Physics: Chapter 04 Gravitation - Questions and Answers
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Class 9 Physics Questions and Answers
Chapter 4: Gravitation
1. Newton's universal law of gravitation
All bodies in the universe attract each other. The force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
All bodies in the universe attract each other. The force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The value of G is 6.67 x 10-11 Nm²/kg². The scientist Henry Cavendish determined the value of G for the first time through experiments.
2. What will be the change in the gravitational force of attraction between two bodies
a) The distance between the bodies is doubled?
The mutual force of attraction decreases to 1/ 4 times
b) The distance between the bodies is halved?
The mutual force of attraction becomes 4 times
c) mass of both bodies doubled?
The mutual force of attraction becomes 4 times
d) Mass of one body is doubled?
Mutual forces of attraction become 2 times
3. Is the force of attraction is the same everywhere on the earth's surface? why?
No, The shape of the earth is not a perfect sphere. So the radius of the earth is not the same everywhere
4. At which part of earth a body experience the maximum force of attraction? Why?
At the poles, where the radius of the earth is small
At the equator the radius is large. so the force of gravity is less
A body experience the maximum force of attraction on the surface of the earth
As the body moves up or down from the surface, the gravitational force decreases
At the centre of the earth gravitational force is zero
5. What are the factors influence acceleration due to gravity?
Mass of earth, the radius of the earth, gravitational constant
6. Will the value of acceleration due to gravity (g) be the same everywhere on the earth? why?
No, Since the earth is not a perfect sphere, the radius of the earth is not the same everywhere
The value of g will be maximum at poles where the radius of the earth is small
The value of g will be minimum at the equator where the radius of the earth is large
At the centre of the earth, the value of g is zero
7. If a stone of mass 10 kg another mass 1 kg fall simultaneously from the top of a five-storey building, which one will reach the ground first?
They will reach the ground at the same time. when an object is dropped from a height uniform acceleration is produced in it by the gravitational force of the earth. The acceleration due to gravity does not depend on the mass of the falling object.
8. A stone and sheet of paper are dropped together, the stone recharge first why?
Objects like paper fall slowly to the air resistance
9. Explain Newton's feather and coin experiment.
Newton placed a feather and coin in a long transparent glass tube with closed ends. The tube kept vertically and allow coin and feather to fall freely from the top. Coin reaches first at the bottom. Because of air resistance feather reaches slowly. The experiment was repeated after removing the air inside the tube and it was found that the feather and coin reach the bottom simultaneously. From the experiment, Newton concluded acceleration due to gravity does not depend on the mass of the falling object.
10. Differentiate between mass and weight
mass
Mass is the quantity of matter contained in a body
its value remains constant at all places
it is measured by a common balance
unit is the kilogram (kg)
scalar quantity
weight
the force with which a body has attracted toward the centre of the earth
value changes from place to place due to
the change in the value of ‘g’
it is measured by a spring balance
unit is Newton or kilogram weight
vector quantity
11. What is mean by free fall ? why does a freely falling body experience weightlessness?
If an object is allowed to fall freely, subject only to the gravitational attractive force of the earth.
This is a free fall. In free fall the gravitational force of the earth (weight)will be used to provide acceleration of the object. Therefore the weight of the object will be zero when it is in free fall.
12. Suppose a spring balance with a body suspended from it, is allowed to fall freely. what will be the reading shown by the balance? Why?
The reading will be zero. In free fall the weight of the object is zero. The weight of both spring balance and the body is used to provide the acceleration for free fall
13. During the rotation of giant wheel a person experience loss of weight on descending why?
On the descent of the giant wheel, the man is falling freely under the action of gravity. Weight of the person is used for his acceleration. During free fall it is not possible for a man to exert weight on the giant wheel. So the person experience loss of weight
14. Even though the magnitude of attraction between earth and aeroplane is the same. When the engine of aeroplane fails. Why does the aeroplane fall on earth instead of earth falling on an aeroplane?
Even though the forces are the same, the acceleration produced on the aeroplane and earth is different. Acceleration of Earth, a earth = F/M
As the mass of earth M is huge, the acceleration of earth is negligibly small
Acceleration of aeroplane, a aeroplane= F/m
As the mass of aeroplane is very small as compared to the mass of earth
Acceleration of aeroplane is greater than earth. So when the engine stopped aeroplane falls to earth
15. Gravitation Textual Questions and Answers
Activity – 1
Lifting a small stone to a certain height and then dropping it downwards.
Question 1.
What do you observe?
Answer: The stone falls down
Question 2.
What could be the reason for the falling of the stone?
Answer: Earth’s attraction is the reason
Question 3.
What change takes place in the speed of the stone as it is thrown up?
Answer: Speed decreases
Question 4.
What about the speed when it falls down?
Answer: Speed increases
Question 5.
Did you apply any force on the stone to bring it down?
Answer: No
Question 6.
From where did the stone get the force for the acceleration?
Answer: The force required for acceleration got from earth’s attraction
Activity – 2
The stone tied to a thread is suspending from a spring balance.
Question 7.
What do you observe?
Answer: Spring is stretched downwards
Question 8.
The spring stretched down when the stone was suspended from it. Why?
Answer: Earth’s attraction is the reason
The earth attracts all objects towards its centre. This force of attraction is the force of gravity.
Conclusion:
The attraction between the two bodies is the gravitational force.
The attraction between bodies and earth is a force of gravity.
Question 9.
Write down instances where the force of gravity is felt.
Answer: Mango falling naturally form a mango tree.
The stone which is thrown upwards reaches the ground.
Raindrops falling downwards.
When we open the bottle filled with water and keeps it upside down water flows out.
An object in our hands falls down when it is released.
Activity – 3
Weighing a stone of lower mass and another of higher mass by using a spring balance.
Mass of a body is the amount of matter contained in it
Question 10.
In which case was the reading higher?
Answer: Reading of the spring balance is higher when the stone having greater mass is weighed.
Question 11.
Which of the stones experienced a greater force of attraction of the earth?
Answer: Greater force of attraction of the earth is experienced in stone having greater mass.
Question 12.
On the basis of these observations, find out the factor that influences the force of attraction from the earth.
Answer: The factor is mass
As the distance between the bodies increases gravitational force decreases.
Another factor which influences the gravitational force is the distance between the objects.
16. Complete the table given below, based on Newton’s law of gravitation?
* Two bodies are at a specific distance so as to attract each other. How many times will the mutual force of attraction be if the mass of one of them is doubled?
Answer: Mutual force of attraction doubled
* What if the mass of both the bodies are doubled?
Answer: Force of attraction becomes 4 times.
* What if the distance between the bodies is doubled?
Answer: When the distance is doubled, attractive force reduced as to one fourth (1/4th).
* What happens when the distance between the bodies is halved?
Answer: When the distance is halved, the force of attraction becomes four times.
18. Why two children sitting close to each other do not come closer due to the mutual force of attraction?
Answer: The attractive force between two children sitting close to each other are mutual. The force felt by each of them is equal and very short in magnitude. So they do not come closer to each other.
19. When a stone of mass 50 kg and another of mass 5 kg fall down simultaneously form the top of a five-storey building, which one will reach the ground first?
Answer: They reaches the ground at the same time.
Reason: Acceleration due to gravity does not depend on the mass of the body. It will be the same for all bodies falling to the earth, g = GM/R²
20. Imagine a spring balance is dropped from the top of a building along with a mass of 1 Kg.
What will be reading of the spring balance while falling down? Why?
If the mass takes 5s to reach the ground, find out the height of the building.
What is the velocity of the mass just before it touches the ground?
Answer.
a)Zero. The weight of a freely falling object is fully utilized for it acceleration.
b)S = ut + 1/2 at²
= 0 X 5 + 1/2 X 10 X 5²
= 125 m (value of g is rounded to 10 m/s²
c) v = u + at
= 0+10 X 5
= 50 m/s
21. The resultant gravitation force at the centre of the Earth is zero. Give reason.
Answer: When a body is placed at the centre of the earth, it is attracted towards all directions. Therefore, the resultant force of attraction at the centre of the earth is zero.
22. Will the value of g be the same everywhere on the Earth? Why?
Answer: No. As Earth is not a perfect sphere, the distance from the centre of the Earth to its surface is not the same at different places. greater the distance, the lower the value of g.
23. An iron block of mass 1 Kg falls freely from the top of a building in 2 s.
1. calculate the height of the building. (g = 10 m/s2)
2. If the experiment was repeated at the same altitude as the moon, how long would it take for the iron block to reach the lunar surface? ( g on moon = 1.62 m/s2)
Answer.
a)S= ut + 1/2 at² = 0+ 1/2 x 10 x2² =20 m
b)S= ut + 1/2 at²
20 = 0+ 1/2 x 1.62 x t²
20 =0.81 t²
t² = 20 / 0.81=24.69 ~ 25
t² = 25
t = 5 s
24. If the distance between two objects attracting each other is increased by 4 times, what will be the force of attraction between them?
Answer:
F=Gm1m2/ d2
d= 4d
F = Gm1m2/( 4d)2
=1/16 Gm1m2/ d2
d= 4d
F = Gm1m2/( 4d)2
=1/16 Gm1m2/ d2
The force of attraction will be decreased by 1/16
25. Calculate the weight of an object of mass 40 Kg.
Answer.
W=mg = 40x 9.8 =392 N
26. a.Calculate the mass of a man on the moon whose mass on the Earth is 60 Kg.
b.What will be his weight on the Earth? Will his weight differ on the moon? If so why?
Answer.
a. 60 Kg ( Mass of an object remains the same everywhere in the Universe)
b. His weight on the Earth , W = mg = 60 X 9.8 = 588 N
weight of the man on the surface of the moon will be lesser than that on the Earth because the acceleration due to gravity on the moon surface is less.
W = mg = 60 X 1.62 = 97.2 N
27. Imagine that a pipe is fixed through the centre of the Earth (B) from the north pole (A) to the south pole (C).
2. What will be the weight of the ball at B?
3. How will the weight of the ball change when it rolls from B to C?
Answer.
1. Force of gravity will decrease.
2. Zero
3. Weight increases because the value of acceleration due to gravity increases as we go outside the centre to the surface of the Earth.
28. At which place on the surface of Earth does an object experiences the maximum force of gravity? Give reason.
Answer.
At poles. That is,
Force of gravity, F = G Mm/R2
The radius of the Earth at the poles is less and hence the force of gravity is high.
29. Two objects of mass 35 Kg and 45 Kg are placed 2 m apart. Find out the force of attraction between them.
Answer.
F = G m1 m2 / d2
= (6.67 X 10-11 X 35 X 45 ) / 4
= 2626.31 X 10-11 N
= 2626.31 X 10-11 N
30. Give the reason - Even though there is a force of attraction between two adjacent objects, they do not come closer to attracting each other.
Answer.
Gravitational force is a very weak force and hence it is unable to overcome the frictional and other force. Therefore two adjacent objects do not come closer to attracting each other.
31. Consider two stones of mass 5 Kg and 50 Kg placed somewhere 500 m above the Earth’s surface.
1. Which stone will experience more force of attraction by the Earth? Why?
2. If both the stones are allowed to fall freely at the same time, which one will reach the Earth’s surface first? Give reason.
3. Compare the weight of the stones while falling down.
Answer.
50 g
F = GMm / R2, the force of gravitation is directly proportioning to the mass of the object.
Both the stones will fall at the same time. Weight of objects falling down will be utilized for acceleration or in other words, the force of gravity does not depend upon the mass of objects.
32. What do you mean by Gravitational force of attraction? On what factors does the force of gravity depend?
Answer.
The Earth attracts all objects towards its centre. This force of attraction is called Gravitational force. The force of gravity depends upon the mass of objects, the mass of the Earth and the distance between the Earth and the object.
33. How the mass and distance between objects affect gravitational force?
Answer:
Among the two bodies if the mass of one body is doubled gravitational force becomes two times. When the mass of both the bodies are doubled. Gravitational force becomes four times. When the distance increases gravitational force decreases. Gravitational force varies in reciprocal of the square of the distance between the bodies.
34. Mass and weight of a body is determined at the pole and at the equator
a) Is there any difference in the mass?
b) Is there any change in the weight?
c) Justify the answer
Answer:
a) No change in mass
b) Change will occur
c) Mass is the quantity of matter present in a matter. Value of g is greater at the poles and lesser at the equator. So the weight varies. Value of g does not affect the mass
35. a) What is mean by free fall?
b) Weight of an object in free fall is zero? Why.
c) If a bottle having a hole at the bottom filled with water falling down freely water will not flow out. Why?
Answer:
a) Falling of a body towards earth due to force of gravity is said to be free fall.
b) In free fall the object cannot be able to give reaction force. Also, the gravitational force is utilized to give acceleration to the object.
c) During free fall, the acceleration of water and bottle remains the same. So no reaction force feel in the water.
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