CBSE Class 9 Physics Gravitation Worksheet

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 shows 1/6 times the weight of the body on the earth at the surface of the moon.

Question. The 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 ∝ m₁ m₂, and F ∝ 1/2 d²
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. 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. The gravitational force acts on all objects in proportion to their masses. Why, then, a heavy object does not fall faster than a light object?
Answer. Acceleration due to gravity does not depend on mass of object. Hence, all bodies fall with the same acceleration provided there is no air or other resistance.

Question. Is value of “g” same at all places on the Earth? Give reason for your answer.
Answer. No, the value of “g” is maximum at the poles and minimum at equator. This variation is because to the oblong shape of the Earth and its rotation about its own axis.

Question. Define the weight of an object on Moon.
Answer. The weight of an object on the Moon is the force with which the Moon attracts that object.

Question. Why does a mug full of water feel lighter inside water?
Answer. A mug of water appears lighter inside the water because a buoyant force acts on the mug when placed inside the water.

Question. What is the importance of universal law of gravitation?
or
Write four phenomenons which were successfully explained using universal law of gravitation.
Answer. Many unconnected phenomenon can be explained by gravitational law successfully.
(i) Force bind us with Earth
(ii) Motion of Moon around Earth
(iii) Motion of planet around Sun
(iv) Tides due to the Moon and Sun

Question. . What is the relation between gravitational force of the Moon with the Earth.
Answer. The gravitational force of the Moon is about one-sixth of what it is on the Earth.

Multiple Choice Question Class 9 Science Chapter 10 Gravitation

1. Two objects of different masses falling freely near the surface of moon would
(a) have same velocities at any instant
(b) have different accelerations
(c) experience forces of same magnitude
(d) undergo a change in their inertia

2. The value of acceleration due to gravity
(a) is same on equator and poles
(b) is least on poles
(c) is least on equator
(d) increases from pole to equator

3. The gravitational force between two objects is F. If masses of both objects are halved without changing distance between them, then the gravitational force would become
(a) F/4
(b) F/2
 (c) F
(d) 2 F

4. A boy is whirling a stone tied with a string in an horizontal circular path. If the string breaks, the stone
(a) will continue to move in the circular path
(b) will move along a straight line towards the centre of the circular path
(c) will move along a straight line tangential to the circular path
(d) will move along a straight line perpendicular to the circular path away from the boy

5. An object is put one by one in three liquids having different densities. The object floats with 1/9, 2/11 and 3/7 parts of their volumes outside the liquid surface in liquids of densities d1, d2 and d3 respectively. Which of the following statement is correct?
(a) d₁ > d₂ > d₃
(b) d₁ > d₂ < d₃
(c) d₁ < d₂ > d₃
(d) d₁ < d₂ < d₃

6. In the relation F = G M m/d², the quantity G
(a) depends on the value of g at the place of observation
(b) is used only when the earth is one of the two masses
(c) is greatest at the surface of the earth
(d) is universal constant of nature

7. Law of gravitation gives the gravitational force between
(a) the earth and a point mass only
(b) the earth and Sun only
(c) any two bodies having some mass
(d) two charged bodies only

8. The value of quantity G in the law of gravitation
(a) depends on mass of earth only
(b) depends on radius of earth only
(c) depends on both mass and radius of earth
(d) is independent of mass and radius of the earth

9. Two particles are placed at some distance. If the mass of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will be
(a) 1/4
(b) 4 times
(c) 1/2 times
(d) unchanged

10. The earth attracts a body of mass 2 kg on its surface with a force of
(a) 9.8 N
(b) 19.6 N
(c) 6.67 x 10^–11 N
(d) 2 x 6.67 x 10^–11 N

11. A stone dropped from a building takes 4 s to reach the ground. The height of the building is
(a) 19.6 m
(b) 80.4 m
(c) 78.4 m
(d) 156.8 m

12. If ge is acceleration due to gravity on earth and gm is acceleration due to gravity on moon, then
(a) g_e = g_m
(b) g_e < g_m
(c) g_e = (1/6)g_m
(d) g_m = (1/6)g_e

13. The mass of a body on the surface of earth is 12 kg. If acceleration due to gravity on moon is 1/6 of acceleration due to gravity on earth, then its mass on moon will be
(a) 2 kgf
(b) 72 kg
(c) 12 kg
(d) zero

14. The atmosphere is held to the earth by
(a) gravity
(b) wind
(c) clouds
(d) earth’s magnetic field

15. The force of attraction between two unit point masses separated by a unit distance is called
(a) gravitational potential
(b) acceleration due to gravity
(c) gravitational field
(d) universal gravitational constant

16. The weight of an object at the centre of the earth of radius R is
(a) zero
(b) infinite
(c) R times the weight at the surface of the earth
(d) 1/R2 times the weight at surface of the earth

Question. . Name the scientist in whose honor the SI unit of pressure is named.
Ans : The SI unit of pressure is named after Blaise Pascal.

Question. . Name the force which accelerates a body in free fall.
Ans : Gravitational force of Earth.

Question. . What is the unit of “g”?
Ans : The unit of “g” is ms–2.

Question. . Why value of “g” more or less constant on or near the Earth?
Ans : Radius of Earth does not change much; “g” is more or less constant on or near the Earth.

Question. . What is weightlessness?
Ans : A body is said to be in a state of weightlessness when the reaction of the supporting surface is zero or its apparent weight is zero.

Q1. State the universal law of gravitation.

Ans. Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. The force is along the line joining the centres of two objects

Q2. Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.

Ans. The gravitational force between the earth and an object on its surface

M_e = Mass of the earth

 Q3. What do you mean by free fall?

Ans. Whenever an object falls toward earth under the force of gravity one and no other force is present, the motion of object is said to be “free fall”.

Q4. What do you mean by acceleration due to gravity?

Ans. The acceleration of free fall is the acceleration due to gravity. We can also say the acceleration of an object due to gravitational force of earth acting on it is known as acceleration due to gravity.

 Q5. Why is the weight of an object on the moon 1/6th its weight on the earth?

Ans. The weight of an object depends on ‘g’ acceleration due to gravity, and the value of ‘g’ on earth: and moon is not same.

 g=Gm/R²

The mass and radius of the earth is more than the mass and radius of the moon.

As W=GMn/R² the weight of a body on the earth is 6 times more than the weight of a same body on moon.

 Q6. Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object?

Ans. The heavy object when falls, the acceleration due to gravity ‘g’ is acting which is independent of the mass of the body.

 g=Gm/R²

Gravitation force is 

∴ F and g are different.

 Q7. What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? [Mass of the earth is 6 x 10²⁴ kg and radius of the earth is 6.4 x 10⁶ m].

Ans. The magnitude of the gravitational force between earth and an object is given by the formula.