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Gravitation Class 9 Question and Answer

In this post, we have presented a set of Gravitation Class 9 Question and Answer. Students of class 9 of different boards (like ICSE, CBSE, and states) will find these useful. Later, we added a section of gravitation class 9 extra questions as well.

Gravitation Class 9 Question and Answer

Q1. As the altitude of a body increases, do the weight and mass both vary?

Answer:

The weight of the body varies with altitude, whereas the mass of an object remains constant.

Q2. If the same body is taken to places having different gravitational field strengths, then what will vary: its weight or mass?

Answer:

The weight of the body varies as the mass remains constant.

Q3. What is the value of gravitational constant G

a) on the earth

b) on the moon

Answer:

The value of gravitational constant G on the earth as well as on the moon = 6.67× 10-11 Nm2/kg2.

The value of G always remains constant irrespective of the location.

Q4. Give the formula for the gravitational force F between two bodies of masses M and m kept at a distance d from each other.

Answer:

The gravitational force F between two bodies of masses M and m kept at a distance d from each other is given as:

F = G × (m × M)/d2

Where G = 6.67× 10-11 Nm2/kg2

Q5. When an object is dropped from a height, it accelerates and falls down. Name the force which accelerates the object.

Answer:

The force that accelerates the object is the gravitational force of the earth.

Q6. On what factor/factors does the weight of a body depend?

Answer:

The weight of a body depends on the mass and on the acceleration due to gravity which varies from place to place.

Q7. What is the weight of a 10 kg mass of the earth? When g = 9.8 m/s2.

Answer:

Weight, w = mg = (10) (9.8) = 98N

Q8. If the mass of an object is 100 kg, what is its weight?

Answer:

Weight, W = mg = (100)(9.8) = 980 N

Q9. The weight of a body is 150N. What is its mass?

Answer:

Weight, W = mg

Mass, m = W/g = 150/9.8 = 15.3 kg

Q10. A body has a weight of 10 kg on the surface of the earth. What will be its weight when taken to the centre of the earth?

Answer:

At the centre of the earth, the value of g is zero. Therefore, the weight of the body at the centre of the earth will be zero.

Q11. Write down the weight of a 50 kg mass on the earth.

Answer:

Weight, W = mg

= (50)(9.8)

= 490 N

Q12. If the weight of a body on the earth is 6N, what will it be on the moon?

Answer:

The weight of the body on the moon will be one-sixth that of the earth. Therefore, the weight of the body on the surface of the moon will be 1N.

Q13. State whether the following statements are true or false.

a) A falling stone also attracts the earth.

b) The force of gravitation between two objects depends on the nature of the medium between them.

c) The value of G on the moon is about one-sixth of the value of G on the earth.

d) The acceleration due to gravity acting on a freely falling body is directly proportional to the mass of the body.

e) The weight of an object on the earth is about one-sixth of the weight on the moon.

Answer:

a) True

b) False

c) False

d) False

e) False

Q14. Does the acceleration produced in a freely falling body depend on the mass of the body?

Answer:

No, the acceleration produced in a freely falling body does not depend on the mass of the body.

Q15. State Kepler’s law which is represented by the relation r3∝ T2.

Answer:

The above relation is used in Kepler’s law of periods which states that the cube of the mean distance of a planet from the sun is directly proportional to the square of times it takes to move around the sun.

Q16. Fill in the blanks with suitable words:

a) The acceleration due to gravity on the moon is about ………. of that on the earth.

b) In order that the force of gravitation between two bodies may become noticeable and cause motion, one of the bodies must have an extremely large ………….

c) The weight of an object on the earth is about ………… of its weight on the moon.

d) The weight of an object on the moon is about ……….. of its weight on the earth.

e) The value of g on the earth is about ……….. of that on the moon.

f) If the weight of a body is 6N on the moon, it will be about ………… on the earth.

Answer:

a) One-sixth

b) Mass

c) Six times

d) One-sixth

e) Six times

f) 36N

Q17. a) What do you mean by the term free fall?

b) During a free fall, will heavier objects accelerate more than the lighter ones?

Answer:

a) The term free fall refers to the falling of an object towards the earth from a certain height towards the earth under the gravitation force of the earth.

b) Acceleration of an object is independent of the mass of the body during free fall.

Q18. Can we apply Newton’s third law to the gravitational force? Explain your answer.

Answer:

Yes, we can apply Newton’s third law to the gravitational force.

The earth exerts a force of attraction on an object such that the force exerted by the other object on the earth is equal and in the opposite direction.

Q19. Give a reason for the following:

The force of gravitation between two cricket balls is extremely small, but that between a cricket ball and the earth is extremely large.

Answer:

We know that the force of gravitation between two bodies is directly proportional to the product of their masses. We also know that the mass of the cricket ball is very small when compared to Earth. That’s why the force of gravitation between the two cricket balls is extremely small, and that between the ball and the earth will be extremely large.

Q20) Describe how the gravitational force between two objects depends on the distance between them.

Answer:

The gravitational force F between the two bodies of masses M and m is kept at a distance d from each other is represented as:

F = G (m.M/d2)

As the force between the two bodies is inversely proportional to the square of the distance between them, which is given as:

F ∝ 1/d2

Therefore, as the distance between the two bodies is doubled, the gravitational force becomes one-fourth and when the distance is halved, the gravitational between these two bodies becomes four times.

Q21. What happens to the gravitational force between two objects when the distance between them is:

a) Doubled?

b) Halved?

Answer:

a) When the distance between the two objects is doubled, the gravitational force becomes one-fourth.

b) When the distance between the two objects is halved, the gravitational force becomes four times.

Gravitation Class 9 Extra Questions

Q1. Calculate the force of gravitation between two objects of masses 50kg and 120kg respectively kept at a distance of 10m from one another.

Answer:

F = G. (m.M/d2)

m = 50 kg

M = 120 kg

d = 10 m

G = 6.7 × 10-11 Nm2/kg2

Substituting the values,

F = 4.02 × 10-9 N

Q2. The mass of the sun is 2 × 1030 kg, and the mass of the earth is 6 × 1024 kg. If the average distance between the sun and the earth be 1.5 × 108 km, calculate the force of gravitation between them.

Answer:

Distance, d = 1.5×108 km = 1.5×1011m

Mass of the sun, m = 2×1030 kg

Mass of the earth, M = 6×1024 kg

Force of gravitation, F = G(m.M/d2)

F = 3.57×1022 N

Q3. What is the force of gravity on a body of mass 150 kg lying on the surface of the earth? Assume mass of the earth = 6 × 1024 kg, radius of the earth = 6.4 × 106m, and G = 6.7 × 10-11 Nm2/kg2.

Answer:

F = G.(m.M/d2)

Force due to gravity,

F = 1472N

Q4. A stone falls from a building and reaches the ground in 2.5 seconds later. How high is the building?

Answer:

Initial velocity, u = 0 m/s

Acceleration due to gravity, g = 9.8 m/s2

Time taken to reach the ground, t = 2.5 s

Height, h = ?

s = ut + ½ gt2

s = 30.65 m

Q5. A piece of stone is thrown vertically upwards. It reaches the maximum height in 3 seconds. If the acceleration of the stone is 9.8 m/s2 directed towards the ground, calculate the initial velocity of the same with which it is thrown upwards.

Answer:

Initial velocity of the stone, u = ?

Final velocity of stone, v = 0

Acceleration due to gravity, g = -9.8 m/s2

Time, t = 3s

Using the relation, v = u + gt
=> u = v – gt

We get, u = 29.4 m/s

Q6. A stone is dropped from a height of 20m.

a) How long will it take to reach the ground?

b) What will be its speed when it hits the ground? g = 10 m/s^2

Answer:

a) Using the relation:

s = ut + ½ gt2

u =0

t2 = 20/5 = 4

t = 2s

b) For a freely falling body:

v2 = u2 + 2gh

v2 = 400

v = 20 m/s

Q7. A stone is thrown vertically upwards at a speed of 20m/s. How high will it go before it beings to fall?

Answer:

Initial velocity, u = 20 m/s

Final velocity, v = 0

Acceleration due to gravity, g = -9.8 m/s2

Height, h = ?

Using the relation v2 = u2 + 2gh

h = 20.4m

Q8. When a cricket ball is thrown vertically upwards, it reaches a maximum height of 5 meters.

a) What was the initial speed of the ball?

b) How much time is taken by the ball to reach the highest point? (use g =10m/s^2)

Answer:

Initial velocity, u = ?

Final velocity, v = 0

Acceleration due to gravity, g = -10 m/s2 (as retardation)

Height, h = 5m

a) For a freely falling body:

v2 = u2 + 2gh

=> 0 = u2 + 2(-10)5

=> u2 = 100

u = 10 m/s

b) Using relation, v = u + gt

=>0 = 10 +(-10)t
=>10t =10

t = 10/10= 1s

See also  The formula for acceleration due to gravity at height h - with derivation
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