# Exam Question for Class 9 Science Chapter 10 Gravitation

Please refer to below Exam Question for Class 9 Science Chapter 10 Gravitation. These questions and answers have been prepared by expert Class 9 Science teachers based on the latest NCERT Book for Class 9 Science and examination guidelines issued by CBSE, NCERT, and KVS. We have provided Class 9 Science exam questions for all chapters in your textbooks. You will be able to easily learn problems and solutions which are expected to come in the upcoming class tests and exams for standard 10th.

## Chapter 10 Gravitation Class 9 Science Exam Question

All questions and answers provided below for Exam Question Class 9 Science Chapter 10 Gravitation are very important and should be revised daily.

**Exam Question Class 9 Science Chapter 10 Gravitation**

**Very Short Answer Type Questions**

**Question: State the Kepler’s third law.****Answer :** The square of the time period of revolution of a planet is proportional to the cube of the semi-major axis of the ellipse traced out by the planet

**Question:** **What is the centripetal force?****Answer :** The force required for a body to move in a circular path is called centripetal force.

**Question:** **Why is G called the universal gravitational constant?****Answer : **The constant G is universal because it is independent of the nature and size of bodies, the space where they are kept and at the time at which the force is considered.

**Question:** **At what place on the earth’s surface is the weight of a body maximum?****Answer: **At the poles, weight of the body is maximum.

**Question:** **What is the ratio of the force of attraction between two bodies kept in air and the same distance apart in water?****Answer :** 1, because the gravitational force does not depend on the nature of the medium.

**Question:** **The weight of a body is 50 N. What is its mass? (g = 9.8 m s–2)****Answer :** Here, W = 50 N

g = 9.8 m s^{–2}

we have, mass =W/g = 50 N/9 .8 ms^{-2}

Mass = 5.10 kg

**Question: Which is more fundamental-mass or weight of a body?Answer:** Mass is more fundamental than weight because the mass of a body remains constant while its weight changes from place to place due to change in the value of g.

**Question:** **Newton’s law of gravitation is also called inverse square law. Why is it so called?****Answer:** According to Newton’s law of gravitation, the magnitude of the gravitational force between two objects is inversely proportional to the square of the distance between them, i.e., F ∝ 1/r ^{2} . That is why this law is also called inverse square law.

**Question:** **Suppose gravity of Sun suddenly becomes zero, then in which direction will the Earth begin to move if no other celestial body affects it?****Answer:** The Earth will move in the direction of the tangent to the Sun orbit.

**Question:** **Define universal gravitational constant.****Answer: **The universal gravitational constant is the force of attraction between two point masses of 1 kg each kept at a distance of 1 m.

**Short Answer Type Questions **

**Question: How does the gravitational force between two bodies changes if the distance between them is tripled? **** Answer:**

**Question:** T**he force of attraction between the two bodies depend upon their masses and distance between them? A student thought that two** **bricks tied together would fall faster than a single one under the action of gravity. Do you agree with his hypothesis or not? Comment.****Answer:** As is known from Newton’s law of gravitation, force of attraction (F) between two bodies is directly proportional to the product of their masses, and inversely proportional to the square of the distance between their centres.

i.e., F ∝ m_{1} m_{2}, and F ∝ 1/d^{2}

The hypothesis of the student is wrong. This is because acceleration due to gravity, with which brick/bricks fall does not depend upon mass of the bricks. So the two bricks tied together would not fall faster than a single one.

**Question:** **Find the ratio between the values of acceleration due to gravity at a height 1 km above and at a depth of 1 km below the earth’s surface.****(radius of earth is R)****Answer:**

**Question:** **Suppose gravitational pull varies inversely as nth power of the distance. Show that the time period of a planet in circular orbit of radius R** **around the sun will be proportional to R(n+1)/2.****Answer: **

**Question:** **Out of aphelion and perihelion, where is the speed of the earth more and why?****Answer:**

**Question:** **Can a body have mass but no weight?****Answer:** Yes, mass of a body is constant wherever it is taken.

But weight of body, W = mg. The weight will be zero where g = 0 e.g. at the centre of Earth.

**Question:** **Two objects of masses m and 2m having the same size are dropped simultaneously from heights h1 and h2 respectively. Find out the ratio of time they would take in reaching the ground.****Answer:**

**Question:** **What is the acceleration of free fall?****Answer**: When a body is released from a height, it begins to fall freely due to earth’s gravitational force acting on it. Due to this force, the body experiences an acceleration towards centre of earth. This acceleration is the acceleration of free fall and is called the acceleration due to gravity.

**Question:** **Give the difference between g and G.****Answer:**

**Question:** **When does an object show weightlessness?****Answer:** Weightlessness is a state when weight of an object is zero. It occurs when a body is in a state of free fall under the effect of gravity alone.

**Question:** **There are two kinds of balances i.e., a beam balance and a spring balance. If both the balances give the same measure of a given** **body on the surface of the earth, will they give the same measure on the surface of the moon?****Explain.****Answer:** Beam balance measure the mass of a body. Since mass of a body remains constant so the beam balance will give the same measure on the surface of the earth and on the surface of the moon.

On the other hand, spring balance measures the weight of a body. Weight of the body W = mg. It means, weight of a body depends upon the value of g. Since value of g on the moon = 1/6 times the value of g on the earth, so the spring balance shows1/6 times the weight of the body on the earth

at the surface of the moon.

**Question:** **a. The line that joins the Saturn to the Sun sweeps areas A _{1}, A_{2} and A_{3} in time intervals of 6 weeks, 3 weeks and 2 weeks respectively as shown in the figure. What is the correct relation between A_{1}, A_{2} and A_{3}? **

**b. The time period of a planet of a star is 8 hours. What will be time period if the separation between the planet and the star is increased to 9** **times the previous value?****Answer: **

**Question:** **What is the source of centripetal force that a planet requires to revolve around the sun? On what factors does that force depend?****Answer:** Gravitational force. This force depends on the product of the masses of the planet and sun and the distance between them.

**Question:** **In which direction do the following forces act when an object is in motion?****a. Frictional forceb. Gravitational forcec. Centripetal force **

**Answer:**(a) Backward direction of relative motion

(b) Downward direction

(c) Towards the centre of the circle

**Question:** **The earth is acted upon by gravitation of Sun, even though it does not fall into the Sun. Why?****Answer:** The earth does not fall into the sun due to gravitational pull of sun on earth. This is because earth is not stationary.

It is revolving around the sun in a particular orbit. The centripetal force required by the earth for revolution around the sun is provided by gravitational pull of sun on the earth.

In other words, gravitational pull of sun on earth is spent up in providing necessary centripetal force required by the earth for revolution around the sun. Hence the earth does not fall into the sun.

**Question:** **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?****Answer**:

**Question: Calculate the average density of earth in terms of g, G and R.****Answer:**

**Question:** **Two uniform solid spheres of radii R and 2R are at rest with their surfaces just touching. Find the force of gravitational attraction between them if density of spheres be ρ?****Direction : Read the passage given below and answer the following questions:****Earth attracts every object towards its centre.****Therefore, every object, when free, falls towards the earth due to gravitational force of earth on it. This is the phenomenon of free fall. The** **acceleration produced in a body in free fall due to gravitational force of earth on the body is called acceleration due to gravity (g). On the** **surface of earth, g = 9.8 m/s2. This value does not depend on mass of the body or nature of the body. ****Answer:**

**Question:** **How does gravity differ from gravitation?****Answer:** The force of attraction between any two objects by virtue of their masses is called gravitational force, whereas the force of gravitation exerted by a huge heavenly body on a smaller object near its surface is called its gravity or the force of gravity.**Question:** **Convert the value of g in km/h2****Answer:**

**Question:** **The weight of any person on the moon is about 1/6 times that on the earth. He can lift a mass of 15 kg on the earth. What will be the** **maximum mass, which can be lifted by the same force applied by the person on the moon?****Answer:**

**Question:** **Is the value of g on moon the same as on earth?****Answer:** No, the value of g on moon is only 1/6 th of its value on earth.

**Long Answer Type Question**

**Question:** **A ball thrown up is caught back by the thrower after 6 s. Calculate a. the velocity with which the ball was thrown up, b. the maximum height attained by the ball and c. the distance of the ball starting from the highest point after 2 s. Take, g = 10 m s– ^{2}.**

**Answer:**

**Question:** **An object weighs 294 N on the earth.****a. What are the differences between the mass of an object and its weight?****b. What would be its mass on the moon?****c. What is the acceleration due to gravity on the moon?****Answer:**

**Question:** **Derive expression for force of attraction between two bodies and then define gravitational constant.****Answer:** From Newton’s law of gravitation, every body in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Let us consider two bodies A and B of masses m_{1} and m_{2} which are separated by a distance r. Then the force of gravitation (F) acting on the two bodies is given by