 # Class 9 Science Important Questions for Gravitation

## Very Short Answer type Questions

1.State the significance of universal law of gravitation.
2.The value of gravitational constant G on earth is $6.67 \times 10^{-11} \ Nm^2/kg^2$. What is its value on the surface of moon?
3. Two objects of masses $m_1$ and $m_2$ are dropped in vacuum from a height above the surface of earth (m1 is greater than m2). Which one will reach the ground first and why?
4.Suppose gravity of earth suddenly becomes zero, then which direction will the moon begin to move if no other celestial body affects it?
5.State the name and type of force which is responsible for holding the solar system together.
6.The factors associated with the motion of an object are: Force, Velocity, Acceleration and Momentum. Out of these four factors which one remains constant for all bodies � large or small undergoing a free fall?
7.Which force is responsible for acceleration of a body in free fall?
8.What will be the acceleration of free fall?
9.A cricket ball thrown vertically upwards, reaches a maximum height of 5 metres. Find the initial speed of the ball. ($g = 9.8 \ m/s^2$)
10.What will be the mass of a body at the centre of the earth as compared to other places on the earth?
11.If the weight of a body on the earth is 6 N, what will it be on the moon? (Given that acceleration due to gravity on moon is one � sixth of that on the earth.)
12.State universal law of gravitation. How the force between the two bodies is affected if the distance between them is tripled?

## Short Answer type Questions

1. �Several phenomena of celestial bodies were believed to be unconnected but universal law of gravitation was successful to explain them�. Mention any two phenomena.
2.A stone and the earth attract each other with an equal and opposite force. Why then we see only the stone falling towards the earth but not the earth rising towards the stone?
3.Give reasons:
(a) Moon does not have atmosphere
. (b) If you jump on the moon, you will rise much higher than if you jump on the earth.
4. Give reasons
(a) As the altitude increases, how do the weight and mass of the body vary?
(b) A stone resting on the ground has a gravitational force of 20 N acting on it. What is the weight and mass of the stone? (Take $g = 10 \ ms^{-2}$)
5.Your mass on earth is 50 kg. Planet m has two times force of gravity of that on earth. What will be your mass and weight on planet m?
6.What is the source of centripetal force that a planet requires to revolve around the sun? on what factors does that force depend?
7.All the planets are moving in circular orbits. What provides the necessary force for this motion and what is the direction of this force? Write the name of this force. What will happen if this force disappears suddenly?
8. What happens to the gravitational force between two objects if the mass of one object is tripled? Explain with the help of formula.
9. The mass of sun is $2 \times\ {10}^{30}$ kg and the mass of earth is $6 \times\ {10}^{24}$ kg. if the average distance between the sun and the earth be $1.5 \times\ {10}^8$ km calculate the force of gravitation between them. (Take G = $6.7 \times\ {10}^{-11}\ {Nm}^2/{kg}^2$)
10.An object is thrown vertically upwards and rises to a height of 10 m. calculate the velocity with which the object was thrown upwards.
11.A stone thrown vertically upwards reaches the maximum height in 3 m. if the acceleration of the stone be $10 \ m/s^2$, calculate its initial velocity.
12.What is meant by:
(i) Free fall
(ii) Acceleration due to gravity?
13.A stone resting on the ground has a gravitational force of 20 N acting on it. What is the weight of the stone? Find its mass. ($g = 10 \ ms^{-2}$)
14. It is said that the mass of an object remains constant at all places while weight may change. Why?
15.How does weight of an object change on moving from equator to poles? When can the weight of an object be zero?
16. Your mass on earth is 50 kg. Planet 'M' has two times force of gravity on earth. What will be your mass and weight on planet 'M'? ($g = 10 \ ms^{-2}$)
17. Explain with Reason
(a) On moon, man feels lighter than earth.
(b) Mass is scalar while weight is a vector quantity.
18.A man�s weight when taken at the poles is 600 N. will his weight remain the same when measured at the equator? Will there be an increase or decrease in his weight? Explain.
19.The weight of a man on the earth is 100 kg. does the weight on the moon increase or decrease?
20.If gravitation is the power by which all bodies tend towards each other, then why do all bodies tend towards the centre of the earth?
21. What is weightlessness?
22. Amit buys few grams of gold at the poles as per the instruction of one his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought? If not, why?

## Moderate Answer type Questions

1.The weight of a body on the surface of the earth is 392 N. what will be the weight of this body on a planet whose mass is double that of the earth and radius is four times that of the earth?
2. What happens to the magnitude of the force of gravitation between two objects if:
(i) distance between the objects is tripled?
(ii) mass of both object is doubled?
(iii) mass of both objects as well as distance between them is doubled?
3.If the distance between two masses be increased by a factor of 6, by what factor would the mass of one of them hence to be altered to maintain the same gravitational force?
4.A fire cracker is fired and it rises to a height of 1000 m. find the
(i) velocity by which it was released.
(ii) time taken by it to reach the highest point (take $g = 10 \ ms^{-2}$)
5.Obtain a relation between the weight of an object on the surface of earth and that on moon.
6.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?
7.Prove that if a body is thrown vertically upward, the time of ascent is equal to the time of descent.
8.Find the gravitational force between the sun and the earth. The mass of the sun is $2.0 \times 10^{30}$ kg, and the mass of the earth is $6.0 \times 10^{24}$ kg. the distance between the sun and the earth is 1.5 \times 1011 m. ($G = 6.67 \times\ {10}^{-11}\ {Nm}^2\ {kg}^{-2}$).
9.Why Newton�s law of gravitation is called the universal law?
OR
Write four phenomena which were successfully explained using universal law of gravitation.
10. What happens to the gravitational force between two objects if:
(i)The mass of one object is doubled?
(ii)The distance between the objects is doubled?
(iii)The masses of both the objects are doubled?
(iv)The distance between them is halved?
(v)Mass of one of the objects is halved?
11.Explain the force responsible for the following:
(a) Moon revolves around the earth.
(b) Objects lying apart on earth attract each other, yet they do not cling to each other.
12.A flower pot drops from the edge of the roof of a multistoried building. Calculate the time taken by the pot to cross a particular distance AB of height 2.9 m, the upper point A being 19.6 m below the roof.
12. If a ball is thrown straight upwards at a speed of 11 m/sec from balcony, 4 m above the ground, how much time would it take to strike the ground at the base of the balcony?
13.(a) Find the value of acceleration due to gravity at a height of 12,800 km from the surface of the earth. Earth�s radius = 6400 km.
(b) State Newton�s law of gravitation and write the mathematical equation describing it.
14.The value of 'g' on earth�s surface is $9.8 \ m/s^2$. Suppose the earth suddenly shrinks to one � third of its present size without losing any mass. What is the value of g on the surface of shrinked earth?
15.A ball thrown vertically up returns to the thrower after 6 s. find:
(i) The velocity with which it was thrown up.
(ii) The maximum height it reaches.
(iii) Its position after 4 s.
16. Show mathematically that acceleration experienced by an object during free fall is independent of mass of the object.
17.Explain why the value of �g� differs while moving from poles to equator on the surface of the earth. State how would its value change on moving:
(i) Towards the centre of the earth.
(ii) Away from the earth.
18.At what height above the earth�s surface would the value of acceleration due to gravity be half of what is on the surface? Calculate.
19.A car falls off a ledge and drops to the ground in 0.5 s. let $g = 10 \ ms^{-2}$.
(i) What is its speed on striking the ground?
(ii) What is its average speed during the 0.5 s?
(iii) How high is the edge from the ground?
20.Answer the following questions:
(a) State the type of motion shown by a freely falling stone.
(b) When a stone is thrown vertically upwards, its velocity is continuously decreased. Why?
(c) Give an example of a motion in which average velocity is zero, but the average speed is not zero.
21.Communication satellites move in orbits of radius 44,400 km around the earth. Find the acceleration of such a satellite assuming that the only force acting on it is that due to the earth. Mass of the earth = $6 \times\ {10}^{24}$ kg. (G = $6.67 \times\ {10}^{-11}\ {Nm}^2\ {kg}^{-2}$).
22.Find the weight of an object at the height 6400 km above the earth�s surface. The weight of the object at the surface of the earth is 20 N and the radius of the earth is 6400 km.

## Long Answer type Questions

1.(a) Differentiate between acceleration due to gravity and universal gravitational constant. Derive a relation between 'g' and 'G'.
(b) State universal law of Gravitation.
2.How does the weight of an object vary with respect to mass and radius of the earth? In a hypothetical case, if the diameter of the earth becomes half of its present value and its mass becomes four times of its present value, then how would the weight of any object on the surface of the earth be affected?
3.How does the gravitational force between two onjects change when the distance between them is reduced to $\frac{1}{4}$th?
4.(a) Write the formula to find the magnitude of gravitational force between earth and an object on earth�s surface.
(b) Derive how does the value of gravitational force 'F' change between two objects when the
(i) distance between them is reduced to half, and
(ii) mass of one object is increased four times?
5.If the distance between masses of two objects is increased by five units, by what facto would the mass of one of them have to be altered to maintain the same gravitational force? Would there be an increases or a decrease in the mass?
6.(a) A planet has mass five times of earth and radius three times that of earth. Calculate it a acceleration due to gravity, if the value of �g� on earth is $10 \ ms^{-2}$.
(b) Differentiate between 'G' and 'g'.
7.Derive the formula for the gravitational force using the factors on which it depends.

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