Gravitation class 9 physics MCQ & SAQ

Gravitation class 9 ncert  solution:

Q1. State the universal law of gravitation.

Ans: Everybody in this 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 distance between their centers.

Universal Law of Gravitation

 

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: Consider F as the force of attraction between an object on the surface of earth and the earth.

Let mass of earth = M

Mass of object = m.

If radius of the object is comparatively negligible, r ≈ radius of earth = R

F = GMm/R2

 

Page No. 136

Q1. What do you mean by free fall?

Ans: When an object falls with a constant acceleration, under the influence of the force of gravitation of the earth, the object is said to have free fall.

 

Q2. What do you mean by acceleration due to gravity?

Ans: During the course of its free fall, a body accelerates due to the force of gravity acting on it. This acceleration is known as acceleration due to gravity.

 

Page No. 138

Q1. What are the differences between the mass of an object and its weight?

Ans:

undefined

 

Q2. Why is the weight of an object on the moon 1/6th its weight on the earth?

Ans: 

Mass of the moon (M) = 7.4 × 1022 kg

Radius of the moon (R) = 1.74 × 106 m

Gravitational constant (G) = 6.7 × 1011 Nm2kg2

We know that acceleration due to gravity(g) = GM/R2

Also, weight , W = mg

W = GMm/R2

Let the mass of the object be m, Let's its weight of moon be Mm and its radius be Rm

By applying the universal law of gravitation, the weight of the object on the moon will be:

  (1)

Let the weight of the same object on the earth be We. The mass of earth is M and its radius is R.

(2)

Substituting the values in equations (1) and (2) we get:

Wm= 2.431 x 1010G x m

and We = 1.474 x 1011 G x m

On dividing Wm and We we get:

∴ Weight of the object on the moon = its weight on earth

 

Page No. 141

Q1. Why is it difficult to hold a school bag with a strap made of a thin and strong string?

Ans:

• It is difficult to hold a school bag with a thin strap because the pressure on the shoulders is quite large. 

• This is because the pressure is inversely proportional to the surface area on which the force acts. 

• The smaller is the surface area; the larger will be the pressure on the surface. In the case of a thin strap, the contact surface area is very small. Hence, the pressure exerted on the shoulder is very large.

Q2. What do you mean by buoyancy? 

Ans: 

• The upward force exerted by a liquid on an object immersed in it is known as buoyancy.
  

• When you try to immerse an object in water, then you can feel an upward force exerted on the object, which increases as you push the object deeper into water.

Q3. Why does an object float or sink when placed on the surface of water?

Ans: 

• If the density of an object is more than the density of the liquid, then it sinks in the liquid.
  

• This is because the buoyant force acting on the object is less than the force of gravity. 

• On the other hand, if the density of the object is less than the density of the liquid, then it floats on the surface of the liquid. 

• This is because the buoyant force acting on the object is greater than the force of gravity. 

Page No. 142

Q1. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg? 

Ans: Mass is always a constant quantity. Therefore, it cannot be more or less than 42 kg.

 

Q2. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than the other. Can you say which one is heavier and why? 

Ans: The correct answer is that a cotton bag is heavier than an iron bar.

The reason is:

True weight = (apparent weight + upthrust)

• The density of the cotton bag is less than that of the iron bar, so the volume of the cotton bag is more compared to the iron bar. So the cotton bag experiences more upthrust due to the presence of air.

• Therefore in the presence of air, the true weight of a cotton bag is more compared to the true weight of an iron bar.

Page No. 143

 

 

 

 

 

 

 

 

 

 

 

Q1. How does the force of gravitation between two objects change when the distance between them is reduced to half?

Ans: The force of gravitation between two objects of masses m1 and m2 at separation r is given by:

F = (G m1m2)/r2

When the distance between them is reduced to half, the gravitational force.

F' = (G m1m2)/r2

= (G m1m2)/ (r/2)2

= (4 G m1m2)/(r2) = 4F

The gravitational force becomes four times.

 

Q2. Gravitational force acts on all objects in proportion to their masses. Why then, does a heavy object not fall faster than a light object?

Ans: The acceleration due to gravity of a freely falling body is independent of the mass of the falling body. Thus, both heavy and light objects fall with the same acceleration.

 

Q3. 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 × 1024 kg and the radius of the earth is 6.4 × 106 m.)

Ans: Here, mass of the earth, M = 6 x 1024 kg

Mass of the object, m = 1 kg, radius of the earth, R = 6.4 x 106 m, G = 6.7 x 10-11 Nm 2 Kg-2 

The magnitude of gravitational force between the earth and object is given by:

Force of gravitation between them,

 

 

Q4. The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?

Ans: 

• Earth attracts the moon with an equal and opposite force as the moon attracts the earth. 

• This is in accordance with Newton’s law of gravitation according to which gravitational force is the mutual force of attraction which is also equal as per to the third law of motion.

Q5. If the moon attracts the earth, why does the earth not move towards the moon? 

Ans:

• The moon revolves around the earth because gravitational force of earth on moon provides the necessary centripetal force for circular motion.

• The moon also attracts the earth, due to which earth also revolves around the moon in circular orbit, but due to the very large mass of earth, the motion of earth is not observed.

• It may be kept in mind that there is no linear motion of earth towards moon or moon towards earth. The gravitational force between them keeps on changing the direction of motion of the moon or earth in a circular orbit.

Q6. What happens to the force between two objects, if 

(a) the mass of one object is doubled? 

(b) the distance between the objects is doubled and tripled? 

(c) the masses of both objects are doubled? 

Ans: Let masses = m1 and m2, distance between masses = r.

According to universal law of gravitation F ∝ m1m2 and F ∝ 1/r2.

(a) If one mass, say m1 is doubled, and F ∝ m1,  then F gets doubled.

(b) If distance is doubled and F ∝ 1/r2, force becomes one-fourth, i.e. 1/(2)2.

If distance is tripled and F ∝ 1/r2, force becomes one-ninth, i.e. 1/(3)2.

(c) If both masses are doubled and F ∝ m1m2, force becomes 4 times, i.e. (2m1)(2m2) = 4m1m2 

 

Q7. What is the importance of the universal law of gravitation? 

Ans:

Universal law of gravitation has successfully explained the phenomena which were earlier considered to be separate:

(a) It explains about the force which holds us on earth.

(b) It explains the motion of planets around the sun.

(c) It explains the motion of the moon around the earth.

(d) It explains the occurrence of tides in the ocean.

 

Q8. What is the acceleration of free fall?

Ans: When a body falls freely and accelerates at every point of its motion due to gravitational force alone. This is called acceleration of free fall. This acceleration is known as the acceleration due to gravity on the earth’s surface. It’s denoted by ‘g’ and its value is 9.8m/s2 and it’s constant for all objects close to earth’s surface (irrespective of their masses).

 

Q9. What do we call the gravitational force between the earth and an object? 

Ans: Gravity is the name of the gravitational force between earth and the object.

 

Q10. Amit buys a few grams of gold at the poles as per the instruction of one of 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? [Hint: The value of g is greater at the poles than at the equator.] 

Ans: Earth is not a perfect sphere. It is flattened at the poles. Thus, the value of ‘g’ is greater at poles than at equator (as g ∝ 1/r2).  This means the weight of gold bought at poles becomes lesser at the equator and the friend does not agree with the weight.

 

Q:11. Why will a sheet of paper fall slower than one that is crumpled into a ball?

Ans: 

• Sheet crumpled into a ball has a small surface area as compared to that of an unfolded sheet. Therefore, the unfolded sheet will experience more friction due to air as compared to the crumpled ball in spite of the same force of gravity acting upon them.

• The larger friction of air slows down the unfolded sheet so it will fall slower than the sheet crumpled into a ball.

Q12. Gravitational force on the surface of the moon is only 1/6 as strong as gravitational force on the earth. What is the weight in newtons of a 10 kg object on the moon and on the earth? 

Ans: Here, mass of object, m = 10 kg, g on the earth’s surface = 9.8 ms-2 

Weight of the object on the earth’s surface = mg

= 10 kg x 9.8 ms-2

= 98.0 kg ms-2 = 98 N

The weight of object on moon is 1/6th that of object on earth, so the weight of the object on the moon:

= 1/6 x 98 N= 16.34 N.

 

 

 

 

 

 

 

 

 

 

Q13. A ball is thrown vertically upwards with a velocity of 49 m/s. Calculate 

(a) the maximum height to which it rises.

(b) the total time it takes to return to the surface of the earth. 

Ans: 

(a) Initial velocity, u = 49 ms-1, Final velocity, v = 0

Maximum height, H = ?

Acceleration due to gravity, g = + 9.8 ms-2 (downward)

As object is moving upward, so a = -gFrom equation  v2 = u2 + 2as , we have a = -g , s = H

 v2 = u2 - 2gH

⇒ H = 122.5 m

(b) When ball returns on the surface of earth, displacement, s = 0

∴ Relation, s = ut + at2 gives s = ut -gt2

0 = (49 ms-1) t -x 9.8 t2

⇒ t (49 -4.9 t) = 0

t = 0 or 49 - 4.9 t = 0 ⇒ t =

As t ≠ 0, so t = 10s

The maximum height is 122.5 m and total time is 10 s.

 

Q14. A stone is released from the top of a tower of height 19.6 m. Calculate its final velocity just before touching the ground. 

Ans: Initial velocity, u = 0, height, h = 19.6 m and g = 9.8 ms-2

Final velocity,

= 19.6 ms-1.

 

Q15. A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s2, find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone?

Ans: Initial velocity, u = 40 ms-1

g = -10 ms-2 (upward motion)

Final velocity, v = 0

During upward motion, g = -10 ms-2

Net displacement on returning back = zero

Total distance = 80 m + 80 m = 160 m

 

Q16. Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = 6 × 1024 kg and of the Sun = 2 × 1030 kg. The average distance between the two is 1.5 × 1011 m. 

Ans: Mass of earth, Me = 6 x 1024 kg

Mass of sun Ms = 2 x 1030 kg and

Distance, r = 1.5 x 1011 m

Gravitational constant, G = 6.67 x 10-11 Nm2/kg2

Force,

= 3.56 x 1022 N

 

Q17. A stone is allowed to fall from the top of a tower 100 m high and at the same time, another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet. 

Ans:  u1 = 0

Let the stones meet at a height h above the ground. Stone 1 covers a distance (100 - h); g = 10 m/s2.

(100 - h) = 0 +

⇒ 100 - h = 5 t2 ...(i)

u2 = 25 m/s

Stone 2 covers a distance h

g = - 10m/s2

h = u2t -

⇒ h = 25 t - 5 t2 ...(ii)

From (i) and (ii), we get 

100 - 25 t + 5 t2 = 5 t2 

100 - 25 t = 0

or t = 4 s (Stones meet 4 s after they are thrown)

h = 25(4) - 5(4)2 = 100 - 80 = 20 m above the ground.

 

Q18. A ball thrown up vertically returns to the thrower after 6s. Find

(a) the velocity with which it was thrown up.

(b) the maximum height it reaches.

(c) its position after 4s.

Ans: 

Time of ascend = Time of descending

⇒  Time taken to reach the top, t = 3 s.

⇒  Final velocity, v = 0, g = -9.8 ms-2.

(a) Initial velocity, u = v - gt = 0 - (-9.8) (3)

= 29.4 ms-1

(b) Maximum height reached, h = (v2 - u2)/g = (29.4)2/(2 x 9.8)

= 44.1 m

(c) After 4s, the ball has started falling and has fallen by some distance h' for 1s.

Here, initial velocity u' = 0, t = 1s

g = + 9.8 ms-1

h' = u't + = 0  +

The ball is at a height, (44.1 - 4.9) = 39.2 m above the ground.

 

Q19. In what direction does the buoyant force on an object immersed in a liquid act?

Ans: An object immersed in a liquid experiences buoyant force in the upward direction.

 

Q20. Why does a block of plastic released under water come up to the surface of water?

Ans:

• Two forces act on an object immersed in water. One is the gravitational force, which pulls the object downwards, and the other is the buoyant force, which pushes the object upwards. 

• If the upward buoyant force is greater than the downward gravitational force, then the object comes up to the surface of the water as soon as it is released within water. 

• Due to this reason, a block of plastic released under water comes up to the surface of the water.

Q21. The volume of 50g of a substance is 20 cm3. If the density of water is 1g cm-3, will the substance float or sink?

Ans: 

• If the density of an object is more than the density of a liquid, then it sinks in the liquid. On the other hand, if the density of an object is less than the density of a liquid, then it floats on the surface of the liquid.

• Here, density of the substance = 

• The density of the substance is more than the density of water (1 g cm−3). Hence, the substance will sink in water.

Q22. The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g cm-3? What will be the mass of the water displaced by this packet? 

Ans:

• Density of the 500 g sealed packet

• The density of the substance is more than the density of water (). Hence, it will sink in water.

• The mass of water displaced by the packet is equal to the volume of the packet, i.e., 350 g.

Gravitation class 9 MCQ :

Question: The mass of an object is

• a) The amount of matter contained in the object, independent of where that object is found

• b) the force of gravity on that object located at a particular point in space

• c) dependent on whether the object is on the Earth or elsewhere.

• d) divided by the Earth acceleration due to gravity to equal force.

Answer: The amount of matter contained in the object, independent of where that object is found

 

Question: Which of the following was NOT a contribution of Newtons to science?

• a) the first good experimental measure of G, the gravitational constant of proportionality

• b) the law of universal gravitation

• c) F = ma

• d) explanations of optical phenomena

Answer: the first good experimental measure of G, the gravitational constant of proportionality

 

Question: If the distance between objects increases, then the gravitational force between the objects will:

• a) decrease

• b) none of these

• c) remain same

• d) Increase

Answer: decrease

 

Question: The relation between the weight of an object on the moon (WM) and on the earth (WE)

• a) WM =1/6 (We)

• b) We = 1/6WM

• c) WM= We

• d) WM =6 WE

Answer: WM =1/6 (We)

 

Question: The weight of an object is:

• a) Greater on earth and lesser on moon

• b) Equal on both earth and moon

• c) lesser on earth and greater on moon

• d) none of these

Answer: Greater on earth and lesser on moon

 

Question: The upward force exerted by the liquid displaced by the body when it is placed inside the liquid is called

• a) Buoyant force

• b) Gravitational force

• c) Centripetal force

• d) Force of gravitation

Answer: Buoyant force

 

Question: SI Unit of pressure is

• a) Pascal

• b) Newton

• c) m/s

• d) Dyne

Answer: Pascal

 

Question: The force acting on an object perpendicular to the surface is called

• a) thrust

• b) Pressure

• c) Weight

• d) weight

Answer: thrust

 

Question: The expression for finding the gravitational force of attraction between any two bodies is

• a) F= Gm1 m2/r2

• b) F= G m1m2/r

• c) F= G m1m2/r3

• d) F = Gm1/r

Answer: F= Gm1 m2/r2

 

Question: The force which keeps the body to move in circular motion when acceleration is

• a) Centripetal force

• b) Magnetic force

• c) Force of gravitation

• d) Electrostatic force

Answer: Centripetal force

Question: The time of ascent when measured from the point of projection of a body projected upwards , the

• a) Time of ascent=Time of descent

• b) Time of ascent > Time of descent

• c) Time of ascent < Time of descent

• d) All of the above

Answer: Time of ascent=Time of descent

 

Question: Weight of an object on the surface of the moon is

• a) 1/6 that on the surface of the earth

• b) 1/5 th that on the surface of the earth

• c) 1/3 that on the surface of the earth

• d) 1/2 that on the surface of the earth

Answer: 1/6 that on the surface of the earth

 

Question: The value of acceleration due to gravity at the poles

• a) Is more than at the equator

• b) Same as at the equator

• c) Is less than at the equator

• d) zero

Answer: Is more than at the equator

 

Question: The value of acceleration due to gravity at the highest point of the motion of the body when a body is projected upwards

• a) 9.8 m/s2 downwards

• b) 0 m/s2

• c) 6 m/s2

• d) 9.8 m/s2 upwards

Answer: 9.8 m/s2 downwards

 

Question: The value of acceleration due to gravity of the surface of the earth is

• a) 9.8 m/s2

• b) 6 m/s2

• c) 8 m/s2

• d) 4.9 m/s2

Answer: 9.8 m/s2

Question. A body of mass 1kg is attracted by the earth with a force which is equal to

(a) 9.8N

(b) 6.67x 1011

(c) 1 N

(d) 9.8m/s

Answer :   A

Question. The SI unit of pressure is

(a) Nm2

(b) N/m

(c) N/m2

(d) N2/m2

Answer :   C

Question. Two objects of different masses falling freely near the surface of the moon would

(a) have same velocities at any instant

(b) have different acceleration

(c) experience forces of same magnitude

(d) undergo a change in their inertia

Answer :   A

Question. Choose the correct unit for the relative density among the following:

(a) kg/cm

(b) unitless

(c) kg/cm

(d) kg/m3

Answer :   B

Question. The gravitational force causes

(a) Tides

(b) Motion of moon

(c) None of them

(d) Both a n b

Answer :   D

Question. A girl stands on a box having 60 cm length, 40 cm breadth and 20 cm width in three ways. In which of the following cases, pressure exerted by the box will be

(a) maximum when length and breadth form the base

(b) maximum when breadth and width form the base

(c) maximum when width and length form the base

(d) the same in all the above three cases

Answer :   B

Question. The Earth attracts the moon with a gravitational force of 1020N. The moon attracts the earth with a gravitational force of

(a) Less than 1020N

(b) 1020N

(c) Greater than 1020N

(d) 10-20N

Answer :   C

Question. The gravitational force between two bodies does not depend on

(a) their separation

(b) their masses

(c) the product of their masses

(d) the medium between the two bodies

Answer :   D

Question. An apple falls from a tree because of the gravitational attraction between the earth and the apple. If F1 is the magnitude of the force exerted by the earth on the apple and F2 is the magnitude of the force exerted by the apple on the earth, then

(a) F1 is very much greater than F2

(b) F2 is very much greater than F1

(c) F1 and F2 are equal

(d) F1 is only a little greater than F2

Answer :   C

Question. In the relation F = GM mld2, 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

Answer :   D

Question. A rectangular block of wood of dimensions 40cm x 20cm x 10cm is kept on a table top. The pressure is greatest when it is rested on

(a) equal for all cases

(b) 20cm x 10 cm

(c) 40cm x 10cm

(d) 40cm x 20cm

Answer :   B

Question. When a body is weighed in a liquid, the loss in weight is equal to

(a) weight of liquid displaced by the body

(b) weight of water displaced by the body

(c) weight of body in air

(d) half the weight of body in air

Answer :    A

Question. Two objects of different masses falling freely near the surface of moon would:

(a) Have different accelerations

(b) Undergo a change in their inertia

(c) Have same velocities at any instant

(d) Experience forces of same magnitude

Answer :   D

Question. The atmosphere is held to the Earth by

(a) gravity

(b) wind

(c) clouds

(d) Earth’s magnetic field

Answer :   A

Question. 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 times

(b) 4 times

(c) 1/2 times

(d) Unchanged

Answer:    B

Question. The earth and the moon are attracted to each other by gravitational force. The earth attracts the moon with a force that is:

(a) More than that exerted by the moon

(b) Same as that exerted by the moon

(c) Less than that exerted by the moon

(d) Not related to that exerted by the moon

Answer:    B

Question. The walls of a dam at the bottom are made thick because

(a) it looks attractive

(b) it is conventional

(c) it is convenient

(d) water exerts large pressure at the bottom

Answer:    D

Question. A mass weighing W can float if the upthrust U is related as

(a) U > W

(b) U = W

(c) U < W

(d) U ≥ W

Answer :   A

1. An object weighing 5 N in air, weighs 4.5 N a liquid. The buoyant force experienced by the object is 

(a) (5 + 4.5) N

(b) 4.5/5 N

(c) 5/4.5 N

(d) 0.5 N

► (d) 0.5 N

2. The density of copper is 9 g/cm3. Then the volume of 90 g copper is​

(a) 90 cm3

(b) 9 cm3

(c) 810 cm3

(d) 10 cm3

► (d) 10 cm3

3. A heavy cylinder of length 1 is slowly taken out of a dense liquid. The weight felt as it is taken out of the liquid

(a) will remain the same

(b) increases as it comes out

(c) decreases as it comes out

(d) increases till it attains the weight in air

► (d) increases till it attains the weight in air

4. The constant G

(a) is a very small quantity

(b) is a force

(c) is the same as g

(d) decrease with increasing altitude

► (a) is a very small quantity

5. The value of ‘g’ is minimum:

(a) on hills

(b) in mines

(c) at equator of earth

(d) at poles of earth

► (c) at equator of earth

6. The tidal waves in the sea are primarily due to______.

(a) atmospheric effect of Earth.

(b) gravitational effect of Mars on Earth.

(c) gravitational effect of Moon on Earth.

(d) None of the above.

► (c) gravitational effect of Moon on Earth.

7. The gravitational force between two bodies does not depend on

(a) their separation

(b) their masses

(c) the product of their masses

(d) the medium between the two bodies

► (d) the medium between the two bodies

8. An apple and a stone dropped from certain height accelerates

(a) equally

(b) differently

(c) depending on density

(d) depends on the position of the sun

► (a) equally

9. If the density of the object placed in a liquid is equal to the density of the liquid, the object will:

(a) Float half immersed

(b) Float wholly immersed

(c) Float completely above the liquid

(d) Sink

► (a) Float half immersed

10. Two particles are placed at some distance. If mass of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will become/remain

(a) 1/4 times

(b) 4 times

(c) 1/2 times

(d) unchanged

► (b) 4 times

11. A packet of 400 g and volume 200 cm3 is put in a water tank. The relative density of packet is

(a) 200

(b) 2

(c) 4

(d) 400

► (b) 2

12. At which of the following locations is the value of g largest?

(a) On top of the Mount Everest

(b) On top of Qutub Minar

(c) At a place on the equator

(d) A camp site in Antarctica

► (d) A camp site in Antarctica

13. The magnitude of gravitational force between the earth and 10 kg body is:

(a) 98 N

(b) 90 N

(c) 9.8 N

(d) 100 N

► (a) 98 N

14. An object of weight 500 N is floating in a liquid. The magnitude of buoyant force acting on it is

(a) 500 N

(b) 400 N

(c) 200 N

(d) 100 N

► (a) 500 N

15. Universal law of gravitation does not explain

(a) the tides on earth due to the moon

(b) the motion of electrons in an atom

(c) the motion of planets around the Sun

(d) the motion of the moon around the earth

► (b) the motion of electrons in an atom

16. Acceleration due to gravity varies with

(a) height

(b) depth

(c) shape of the planet

(d) all of the above

► (d) all of the above

17. The seawater is denser than freshwater due to

(a) Mixing of sand

(b) Stagnation

(c) Mixing of salts

(d) Evaporation

► (c) Mixing of salts

18. The universal law of gravitation was postulated by

(a) Galileo

(b) Newton

(c) Copernicus

(d) Archimedes

► (b) Newton

19. Gravitational force does not depend on:

(a) Masses of two bodies.

(b) The gravitational constant.

(c) Charge on two bodies

(d) Separation between two bodies.

► (c) Charge on two bodies

20. The weight of an object______.

(a) is the quantity of matter it contains.

(b) refers to its inertia.

(c) is the force with which it is attracted to the earth.

(d) is the same as the mass.

► (c) is the force with which it is attracted to the earth.

21. Two objects of different masses falling freely near the surface of moon would 

(a) experience forces of different magnitudes

(b) have different accelerations

(c) have same velocities of any instant

(d) undergo a change in their inertia

► (c) have same velocities of any instant

22. The walls of a dam at the bottom are made thick because

(a) it looks attractive

(b) it is conventional

(c) it is convenient

(d) water exerts large pressure at the bottom

► (d) water exerts large pressure at the bottom

23. The same body is immersed in two liquids A and B in succession. The extent to which the body sinks in liquid B is less than in liquid A. What are the conclusions that could be derived from such an observation

(a) The density of liquid A is more then B

(b) The density of liquid B is more than A

(c) The density of solid is less than the liquid in both

(d) No such conclusion can be made

► (b) The density of liquid B is more than A

24. Archimedes’ principle holds for

(a) liquids only

(b) gasses only

(c) liquids and gasses both

(d) neither liquids or gasses

► (c) liquids and gasses both

25. If upthrust U is equal to 1/4  the weight of the object in air, then the weight felt in the liquid is

(a) 1/4W

(b) 3/4W

(c) 1/2W

(d) 2 W

► (b) 3/4W

26. Acceleration due to gravity on the surface of the moon is 1/6th of its value on the surface of the earth as they have different______.

(a) mass

(b) density

(c) radius

(d) All of the above

► (d) All of the above

27. 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

► (d) universal gravitational constant

28. Weight of an object is highest at

(a) Center of earth

(b) Poles

(c) Above the earth’s surface

(d) Equator

► (b) Poles

Gravitation class 9 physics SAQ:

Q.1. (a) Name the balances used to measure mass and weight. 

(b) Weight of an object will be lesser or more at Antarctica as compared to the weight of object at Delhi. Give reason for your answer.    [CBSE 2016]

Ans. (a) Mass is measured by beam balance. Weight is measured by spring balance.

b) Earth is flattened at poles. So the distance of the body from its center is less. As Hence the value of ‘g’ in Antarctica will be more as compared to that in Delhi. Consequently, its weight will be more in Antarctica than in Delhi.

Q.2. Mona weighs 750 N on Earth.

(i) On the planet Mars, the force of gravity is 38% of that of Earth. How will Mona weigh on Mars?

(ii) What will be Mona’s mass on Earth (g = 10 m/s2)?    [CBSE 2016]

Ans. Value of g on Mars, gm = 0.38 times value of g on Earth = 0.38 x 9.8

∴ Weight on Mars = 0.38 x Weight on Earth

= 0.38 x 750 = 285 N

Mass of Earth =

Q.3. The gravitational force between two objects is 100N. How should the distance between these objects be changed so that the force between them becomes 50 N?    [CBSE 2016]

Ans. F = 100 N. When distance between objects in R.

If F becomes (F/2),  then the distance becomes R'.

i.e. distance is increased √2 times.

Q.4. 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?    [CBSE 2016]

Ans.

where Me = Mass of earth,

Re = Radius of earth

Mass of planet = 2Me; Radius of planet = 4Re

∴

Weight on plane

Q.5. “Several phenomena of celestial bodies were believed to be unconnected but the universal law of gravitation was successful to explain them”. Mention any two phenomena.    [CBSE 2016]

Ans. Two phenomena explained by universal law of gravitation are:

(a) motion of planets around the sun in their orbits.

(b) presence of an atmosphere around the earth.

Q.6. (a) Suggest two ways to decrease pressure on a surface.

(b) The density of an object is 1.8 g/cm3. Express it in kg/m3.    [CBSE 2016]

Ans. (a) Two suggested ways so as to decrease pressure on a surface are:

(i) decrease the force acting on the surface, and

(ii) increase the surface area of a given surface.

(b) The density of an object in CGS system is d = 1.8 g/cm3

∵ 1 g/cm3 = 103 kg/m3, hence density in SI system will be

d = 1.8 x 103 kg/m3

Q.7. A steel needle sinks in water kut a steel ship floats. Explain how.    [CBSE 2014, 2016]

Ans. A steel needle sinks in water because the density of steel is more than the density of water. The ship is made up of steel. But the interior of the ship is made hollow by giving it a special shape. When immersed in water, the ship displaces a large volume of water and its effective density becomes less than density of water. As a result, the weight of displaced water i.e., the upward thrust due to water becomes more than the weight of the ship. So, the ship floats and may even carry a lot of cargo.

Q.8. The ratio of gravitational force on Neptune to the gravitational force on earth is 9:8. Ramesh weighs 792 N on earth. Calculate his weight on Neptune.    [CBSE 2016]

Ans. We know that the gravitational force acting on a given object due to a planet is called the weight of that object on the planet.

In present question weight of Ramesh on earth

⇒ Weight of Ramesh on Neptune,

Q.9. 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 on the surface of shrinked earth?    [CBSE 2016]

Ans. If radius of earth be R and mass of earth be M, then

If suddenly earth shrinks to one-third of its present size without any change in mass then  

Hence new value of g will be

Q.10. A man feels lighter on the moon than on earth. Why?    [CBSE 2016]

Ans. Since the value of acceleration due to gravity at the surface of the moon is only 1/6th of its value on the earth, the weight of a person on the moon is only 1/6th of its value on earth. As a result, a man feels lighter on the moon.

Q.11. The gravitational force between two small spheres is F. What will happen if masses of the two spheres, as well as distance between them, is doubled?    [CBSE 2010, 2016]

Ans. Force will remain unchanged.

Q.12. Mass is scalar but weight is a vector. Why?    [CBSE 2016]

Ans. Mass of an object is a measure of its inertia and hence it is a scalar. On the other hand, weight of an object is the gravitational force due to gravity acting on it and is, therefore, a vector quantity.

Q.13. Why is G called a universal constant?    [CBSE 2010, 2011. 2012, 2015, 2016]

Ans. Because the value of G is independent of the two objects or the distance between them or the intervening medium between them.

Q.14. Which force is responsible for the stability of our universe?    [CBSE 2015]

Ans. The force of gravitation.

Q.15. A ball projected vertically upwards with an initial velocity u goes to a maximum height h. What is its velocity at the highest point?    [CBSE 2015]

Ans. Zero.

Q.16. When an object is dropped from a height, it accelerates while falling. Name the force which accelerates the object.    [CESE 2015]

Ans. The force of gravity causes an acceleration in the falling object.

Q.17. What is the force of buoyancy?    [CBSE 2013, 2014, 2015]

Ans. Force acting vertically upward due to a fluid on a solid immersed completely or partly in that fluid.

Q.18. Name the SI unit of pressure.    [CBSE 2014, 2015]

Ans. A pascal (1 Pa), where 1 Pa = 1 N m-2.

Q.19. How is pressure related to thrust?

Ans.

Q.20. An object of mass 2 kg is taken to the surface of the moon. What will be its mass there and why?    [CBSE 2015]

Ans. The mass of the object will remain 2 kg because the mass of an object cannot change.

Q.21. A ball is thrown vertically upwards and reaches a maximum height in 3 s. Find (i) the velocity with which it was thrown upwards, (ii) the maximum height attained by the ball. Take g = 10 m s-2.    [CBSE 2012, 2015]

Ans. Here final velocity of ball at highest point v = 0, time of ascent of ball t = 3 s and a = - g = - 10 m s-2

(i) As per relation v = u + at, we have

0 = u + ( - 10) x 3 ⇒ u = + 30 m s-1

(ii) As per relation v2 - u2 = 2as, we have

(0)2 - (30)2 = 2 x ( - 10) x h ⇒ h = 45 m

Q.22. A particle is released from rest from a height.

(a) Find the distance it falls through in 1 second?

(b) Find the distance it falls through in 3 seconds. Also, find the speed with which it strikes the ground after 3 seconds.    [CBSE 2015]

Ans. Here initial velocity of particle u = 0 and acceleration a = g = 9.8 m s-2.

(a) Distance h1 through which the particle falls in time t1 = 1 s is:

(b) Distance h through which particle falls in time t = 3 s will be:

and speed with which it reaches the ground after time t = 3 s, will be:

v = u + gt = gt = 9.8 x 3 = 29.4 m s-1.

Q.23. If the distance between two masses were increased by a factor of 6, by what factor would the mass of one of them be altered to maintain the same gravitational force?    [CBSE 2015]

Ans.  By law of gravitation, of F ∝ mass

If the distance is increased by a factor of 6, F decreases by 1/36 times.

Thus, mass should be increased by (6)2 times i.e. 36 times.

Thus mass also has to be increased.

 

Q.24. 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 objects is doubled?

(iii) mass of both objects as well as distance between them is doubled?    [CBSE (CCE 2015)]

Ans. (i) If distance is tripled, i.e. R → 3R

By Newton's law,

Thus F becomes (1/3)2 = one-ninth of its original value

(ii) Masses are doubled i.e.

By Newton’s law, F ∝ M1M2

Thus F becomes (2) x (2) = 4 times its original value.

(iii) Masses

Distance

By Newton’s law,

Or

On changing masses and distance,

Thus force remains unchanged.

 

Long Answer Type Questions

Q.1. (a) A body floats in kerosene of density 0.8 x 103 kg/m3 up to a certain mark. If the same body is placed in water of density 1.0 x 103 kg/m3, will it sink more or less? Give reason for your answer.

(b) If a fresh egg is put into a beaker filled with water, it sinks. On dissolving a lot of salt in the water the egg begins to rise and then floats. Why?    [CBSE 2016]

Ans. (a) The body will sink less in water. When a body of weight W floats on a liquid of density p with a volume V immersed in the liquid, the upthrust must balance the weight and hence

⇒

As density of water (pw = 1.0 x 103 kg/m3) is greater than density of kerosene (ph = 0.8 x 103 kg/m3), the volume of the body immersed in water will be less than that in kerosene.

Gravitation class 9 physics long question:

a) State Universal law of gravitation. Write the expression for the gravitational force between the earth and an object lying on the surface of the earth.

(b) Gravitational force plays an important role in nature. Justify this statement.

Or

State universal law of gravitation. Derive an expression for gravitational force between two bodies.

(CBSE 2012)

Answer:

Definition of universal gravitational constant (G)

If m1 = 1 unit, m2 = 1 unit and r = 1 unit, then G = F Thus, universal gravitational constant (G) is defined as the force of attraction between two bodies of unit masses separated by a unit distance.

Units of universal law of gravitation:

Characteristics of Gravitational force

1. Gravitational force between two bodies or objects does not need any contact between them. It means, gravitational force is action at a distance.

2. Gravitational force between two bodies varies inversely proportional to the square of the distance between them. Hence, gravitational force is an inverse square force.

3. The gravitational forces between two bodies or objects form an action-reaction pair. If object A attracts object B with a force F1 and the object B attracts object A with a force F2 (Figure 7), then

4. Gravitational force between two small bodies is very small. On the other hand, the gravitational force between two large bodies (say the sun and the earth) is large.

5. Gravitational force between two objects is always attractive in nature.

What do you understand about free fall ? Derive an expression for the acceleration due to gravity. Write the factors on which “acceleration due to gravity” depends.

Definition of Free Fall

The falling body on which only the force of gravitation of the earth acts is known as a freely falling body and such a fall of a body is known as free fall. A freely falling body has constant acceleration equal to acceleration due to gravity (g), irrespective of its mass.

Thus, all bodies irrespective of their masses fall down with constant acceleration.

Motion of a freely falling body is uniformly accelerated motion.

Consider a body of mass m near the surface of the earth (Figure 11). The force acting on the body is the gravitational force of the earth. The magnitude of the gravitational force acting on the body due to the earth is given

where, M = mass of the earth

R = radius of the earth

[Here, the height of the body from the surface of the earth is neglected as compared to the radius of the earth because R = 6400 km is very large.]

This gravitational force (F) produces acceleration equal to ‘g in the body of mass m. So according to Newton’s second law of motion, which is the expression for the acceleration due to gravity.

Eqn. (3) shows that acceleration due to gravity does not depend on the mass of the object or body. Thus, all objects hollow or solid, big or small should fall at the same rate.