The document Practice Questions, Gravitation, Class 9, Science | EduRev Notes is a part of the Class 9 Course Science Class 9.

All you need of Class 9 at this link: Class 9

**Q1.** **Let us find force of attraction between two block lying 1m apart. Let the mass of each block is 40 kg.****Sol.**

F= ?

m_{1} = 40 kg

m_{2} = 40 kg

d = 1 m

**Q2.** **The gravitational force between two object is 49 N. How much distance between these objects be decreased so that the force between them becomes double?**

**Sol. Step-1 :** Here, F = 49 N

Let r = original distance between the objects.

m_{1} and m_{2} = masses of the objects

Gravitational force between these objects is given by

**Step-2 :** Now, F' = 2F = 2 × 49 N = 98 N

Let, r' = decreased distance between the objects

m_{1} and m_{2} = masses of the objects

Gravitational force between these objects is given by

Dividing (1) by (2), we get

[Q F' = 2F]

**Q3.** **Two bodies A and B having masses 2 kg and 4 kg respectively are seperated by 2m. Where should a body of mass 1 kg be placed so that the gravitational force on this body due to bodies A and B is zero?**

**Sol.** Let a body of mass 1 kg be placed at point P at a distance x from body A. Therefore, the distance of mass 1 kg from a body B = (2 x) m.

Gravitaional force on 1 kg due to body A is given by

F = N(along PA) = N (along PA)

Gravitational force on 1 kg due to body B is

F' = N (along PB).

Since gravitational force on 1 kg due to bodies A and B is zero, therefore,

F = F ' or (2 x)^{2} 2x^{2} or (2 x) = x = 1.414 x

or 2.414 x = 2 or x = 0.83 m.

**Q4. Calculate the force of gravitation due to a child of mass 25 kg on his fat mother of mass 75 kg if the distance between their centres is 1m from each other. Given G = (20/3) × 10 ^{-}^{11} Nm^{2} kg^{-}^{2}.**

**Sol. **Here m_{1} = 25 kg ; m_{2} = 75 kg ; d = 1 m ; G = × 10^{-}^{11} Nm^{2} kg^{-}^{2}

Using F = 12,500 × 10^{-}^{11}

or F = 1.25 × 10^{-}^{7} N

**Q5. A boy drops a stone from the edge of the roof. If passes a window 2m high in 0.1s. How far is the roof above the top of the window ?****Sol.** Let a stone be dropped from the edge of the roof A. Let it passes over B with a velocity say u. Consider motion BC.

u = ?, a = 9.8 ms^{_2} ; s = h = 2m ; t = 0.1 s

Using s = ut gt^{2}, we have

2 = u(0.1) × 9.8 (0.1)^{2}

2 = 0.1u 0.049

0.1u = 2 _ 0.049

or u = = 19.51 ms^{_1}

This initial velocity at B in motion BC is the final velocity in motion AB. Considering motion AB, we have

u = 0 ; v = 19.51 ms^{_1} ; s = ? ; a = 9.8 ms^{_2}

Using v^{2} _ u^{2} = 2as, we have

(19.51)^{2} _ (0)^{2} = 2 × 9.8 s

or s = 19.4 m

Roof is 19.4 m above the window.

**Q****6.** **A ball thrown up is caught by the thrower after 4 s. With what velocity was it thrown up? How high did it go? Where was it after 3 s ? (g = 9.8 m s ^{_2})**

Here u = ?; a = - 9.8 m s

Using v = u at

or 0 = u _ 9.8 × 2

or u = 19.6 m s

Again v

0 _ (19.6)

h = 19.6 m

After 2s, it starts coming downwards (starting with u = 0). Considering downward motion.

u = 0; a = 9.8 m s

s = ut at

or s = 0 × 9.8 (1)

**Q7.** **Coconut is hanging on a tree at a height of 15 m from the ground. A boy launches a projectile vertically upwards with a velocity of 20 m s ^{_1}. After what time the projectile pass by coconut? Explain the two answers in this problem.**

Using s = ut at

15 = 20t (_10) t

Dividing throughout by 5, we have

3 = 4t - t

or t

or (t _ 1) (t _ 3) =t _ 1 = 0 or t = 1s

or t _ 3 = 0 or t = 3s

After 1s, it will cross coconut while going up and after 3 s while coming down.

F

Thus, pressure exerted, P

In Fig. (b), A

F

Thus, pressure exerted, P

**Q9.**** A sealed tin of Coca-cola of 400 g has a volume of 300 cm**^{3}**. Calculate the density of the tin.****Sol.** Here, mass of tin m, = 400 g

Volume of tin, V = 300 cm^{3}

Density of tin, r = 1.33 g cm^{3}.

**Q10.**** A sealed cane of mass 600 g has a volume of 500 cm ^{3}. Will this cane sink in water ? Density of water is 1 g cm^{3}.**

Volume of cane, V = 500 cm

Density of cane, r = 1.2 g cm

Since, density of the cane is greater than the density of water, so the can will sink in water.

**Q11.** **The density of water is 1000 kg m ^{3}. If density of gold is 19320 kg m^{3}, find the relative density of the gold.**

Density of gold = 19320 kg m

Using, R.D. of gold = we get R.D. of gold

**Q12.** **The density of water is 1000 kg m ^{3}. If relative density of iron is 7.874, then calculate the density of iron.**

Relative density (R.D.) of iron = 7.874

Using, R.D. of iron = we get

Density of iron = R.D. of iron × density of water

= 7.874 × 1000 kg m

**Q13.** **A plastic bottle of 500 g has a volume of 450 cm ^{3}. Will the bottle float or sink in water? Density of water is 1 g cm^{3}? Also calculate the mass of the water displaced by the bottle.**

Volume, V = 450 cm

(i) Density of bottle, r = 1.11 g cm

Since, density of bottle is greater than the density of water (1 g cm

(ii) Mass of water displaced by the bottle = Volume of water displaced × Density of water

= Volume of bottle × Density of water

= 450 cm

= 450 g.

**Q14.** **What is the force of gravitation between two point masses of 1 kg and 2 kg kept 1 m apart?****Sol.** m = 1 kg, m_{2} = 2 kg, r = 1 m

F = 6.67 x 10^{_11} × = 13.34 × 10^{_11} N.

This is an extremely small force.

**Q15.** **Calculate the force of gravitation between the earth and the sun.****Mass of Earth = 6 × 10 ^{24} kg, Mass of Sun = 2 × 10^{30} kg. The average distance between the two is 1.5 × 10^{11}m.**

**Q16.** **Write down the expression for acceleration experienced by a particle on the surface of the moon due to gravitational force on the moon. Find the ratio of this acceleration to that experienced by the same particle on the surface of the earth. If the acceleration due to gravity on the earth is 9.8 ms ^{_2}, what is the acceleration of a particle on the moon's surface? Mass of moon = 7.3 × 10^{22} kg; Mass of Earth = 6 × 10^{24} kg. Radius of moon= 1.74 × 10^{6} m, Radius of earth = 6.4 × 10^{6} m.**

Dividing (1) by (2)

= 0.16

g

**Q17.** **Find the value of acceleration due to gravity at a height of (a) 6400 km, (b) 12,800 km from the surface of the earth. Radius of earth is 6400 km.****Sol.** We know that g_{h} =

Also g_{h} = g

(a) g = 9.8 ms^{_2}, R = 6400 km, R h = 6400 6400 = 12800 km

(g_{h})_{A} = 9.8 = 9.8 = = 2.45 ms^{_2}

(b) (g_{h})_{B} = 9.8

= 1.09 ms^{_2}

**Q18.** **A particle is thrown up vertically with a velocity of 50 m/s. (a) What will be its velocity at the highest point of its journey? (b) How high would the particle rise? (c) What time would it take to reach the highest point.****Sol.** At the highest point the velocity will be zero.

Considering activity A to B

Using v = u at

0 = 50 _ 9.8 × t

t = 5.1 s

Also v^{2} _ u^{2} = 2as

0^{2} _ (50)^{2} = 2 (_9.8) × s

5 = 127.5 m

**Q19. With reference to the above sample problem, (a) Find the time the particle takes from the highest point back to the initial point (b) Find the velocity with which the particle reaches the initial point.****Sol.** The data is given in the adjacent figure. Considering activity B to A

Using v^{2} _ u^{2} = 2as

v^{2} _ 0^{2} = 5(9.8)(127.5)

v = 50 m/s

Also v = u at

50 = 0 9.8 (t)

t = 5.1 s

**Q****20.** **A ball is dropped from the top of a tower 40 m high. What is its velocity when it has covered 20 m ? What would be its velocity when it hits the ground? Take g= 10 m/s ^{2}.**

Activity from A to B :

u

v

v

v

v

Activity from A to C : C is a point on the ground

u

v

v

v

v

**Q21.** **A body is thrown up with a speed 29.4 ms ^{_1}.**

v

= 29.4 (_9.8) x 1 = 19.6 ms

(ii) u = 29.4 ms, a = -9.8 ms^{_2}, t_{2} = 2 s, v_{2} = ?

v_{2} = u at_{2}

= 29.4 (_9.8) × 2 = 9.8 ms^{_1}

(iii) u = 29.4 ms^{_1}, a = _9.8 ms^{_2}, t_{3} = 3 s, v_{3} = ?

v_{3} = u at_{3}

= 29.4 (_9.8) × 3 = 0

(b) (i) u = 29.4 ms^{_1}, a = _9.8 ms^{_2}, s_{1} = h_{1} , t_{1} = 1s

h_{1} = ut_{1} at_{1}^{2} =29.4 × 1 (_9.8) × 1 =24.5m

(ii) u = 29.4 ms^{_1}, a = -9.8 ms^{_2}, s_{2} = h_{2}, t_{2} = 2 s

h_{2} = ut_{2} at_{2}^{2} = 29.4 × 2 (_9.8) × 2^{2} = 39.2 m

(iii) u = 29.4 ms^{_1}, a = _9.8 ms^{_2}, s_{3} = h_{3}, t_{3} = 3 s

h_{3} = ut_{3} at_{3}^{2} = 29.4 x 3 (_9.8) × 3^{2} = 44.1 m

**Q22.** **What is the weight of a person whose mass is 50 kg.****Sol.** The weight of the person

W = mg = 50 x 9.8 = 490 N**Note:** The gravitational unit of force is kg-f (kilogram force) or kg-wt (kilogram weight) 1 kg-wt = 9.8 N = 1 kg-f

490 N = 50 kg-f

**Q23.** **Weight of a girl is 294 N. Find her mass.****Sol.** W = mg

294 = m × 9.8

m = 30 kg

**Q24.** **Weight of an object is 294 N on the surface of the earth. What is its weight at a height of 200 km from the surface of the earth. Radius of the earth= 6400 km.****Sol. **Weight at a height of 200 km

W_{h} = mg

Here mg = 294 N, R = 6400 km, h= 200 km

W_{h} = 294

= 276.45 N

Note:Weight decreases with increase of height from the surface of the earth.

**Q25. The gravitational force between two objects is F. How will this force change when****(i) distance between them is reduced to half ?****(ii) the mass of each object is quadrupled ?**

**Sol.** (i) According to Newton's law of gravitation, gravitational force F between two objects distance r apart is

When distance between them is reduced to half, i.e., r' = r/2, the force,

Thus, F' = 4F

i.e., force becomes 4 times its previous value. "

(ii) Again, according to Newton's law of gravitation, the gravitational force F between two'objects of masses m_{1} and m_{2} is

F µ m_{1}m_{2}

When mass of each object is quadrupled,

m'_{1} = 4m_{1}

and m'_{2} = 4m_{2}

The force, F ' µ m'_{1}m'_{2}

or F' = 16 F

i.e., force becomes 16 times its previous value.

**Q****26. A sphere of mass 40 kg is attracted by a second sphere of mass 15 kg when their centres are 20 cm apart, with a force of 0·1 milligram weight. Calculate the value of gravitational constant.****Sol.** Here, m_{1} = 40 kg, m_{2} = 15 kg

From r = 20cm =2 × 10^{_1} m

F = 0·1 milligram weight = 0·1 × 10^{_3} gram weight

= 10^{_4} × 10^{_3} kg wt = 10^{_7} × 9·8 N (1 kg wt = 9.8 N)

From G = 6·53 × 10^{_11} Nm^{2}/kg^{2}

**Q****27. Calculate the force of gravity acting on your friend of mass 60 kg. Given mass of earth = 6 x 1024 kg and radius of Earth = 6·4 x 10 ^{6} m.**

R = 6·4 × 10

G = 6·67 × 10

Thus, F = 58·62 N

**Q****28.** **A particle is thrown up vertically with a velocity of 50 m/s. What will be its velocity at the highest point of the journey? How high would the particle rise ? What time would it take to reach the highest point? Take g = 10 m/s ^{2}.**

final velocity, v = ?

height covered, h = ?

time taken, t = ?

g = 10 m/s

At the highest point, final velocity v = 0

From v

0 _ (50)

h = 125 m

From v = u gt,

or 0 = 50 (_10) t

t = 50/10 = 5 s

**Q****29.** **A force of 15 N is uniformly distributed over an area of 150 m ^{2}. Find the pressure in pascals.**

area, A = 150 cm

Thus, pressure, P =1000 Pa

**Q****30.** **How much force should be applied on an area of 1 cm ^{2} to get a pressure of 15 Pa ?**

pressure, P = 15 Pa = 15 N/m

As P = , F = P × A = (15 N/m

**Q31.** **A block weighing 1·0 kg is in the shape of a cube of length 10 cm. It is kept on a horizontal table. Find the pressure on the portion of the table where the block is kept.****Sol.** Here, force acting on the table, F = 1·0 kg = 10 N

area of the table on which this force acts, A = 10 cm × 10 cm = 100 cm^{2}

= 100 × 10^{_4} m^{2} = 10^{_2} m^{2} (1 cm^{2} = 10^{_4} m^{2})

Pressure on the table, P = 1000 Pa

**Exercise -1 **

**Q1.** If a rock is brought from the surface of the moon,

(A) its mass will change

(B) its weight will change but not mass

(C) both mass and weight will change

(D) its mass and weight both will remain same

**Q****2.** A body is weighed at the poles and then at the equator. The weight

(A) at the equator will be greater than at the poles

(B) at the poles will be greater than at the equator

(C) at the poles will be equal to the weight at the equator

(D) depends upon the object**Q3.** Consider a satellite going round the earth in a circular orbit. Which of the following statements is wrong ?

(A) It is a freely falling body.

(B) It is moving with constant speed.

(C) It is acted upon by a force directed away from the centre of the earth which counter-balances the gravitational pull

(D) Its angular momentum remains constant

**Q4.** A missile is launched with a velocity less than the escape velocity. The sum of its kinetic and potential energy is

(A) positive

(B) negative

(C) zero

(D) may be positive or negative depending upon its initial velocity

**Q5.** SI unit of g is

(A) m^{2}/s (B) s/m^{2} (C) m/s^{2} (D) m/s

**Q6.** SI unit of G is

(A) N^{2}_m^{2}/kg (B) N_m^{2}/kg (C) N_m/kg (D) N -m^{2}/kg^{2}

**Q7.** Choose the correct statement of the following:

(A) All bodies repel each other in this universe.

(B) Our earth does not behave like a magnet.

(C) Acceleration due to gravity is 8.9 m/s^{2}.

(D) All bodies fall at the same rate in vacuum.

**Q8.** Maximum weight of a body is

(A) at the centre of the earth

(B) inside the earth

(C) on the surface of the earth

(D) above the surface of earth

**Q9.** If the distance between two masses be doubled, then the force between them will become

(A) times

(B) 4 times

(C) times

(D) 2 times

**Q10.** A body falls freely towards the earth with

(A) uniform speed

(B) uniform velocity

(C) uniform acceleration

(D) none of these

**Q11.** If the mass of a body is M on the sufrace of the earth, then its mass on the surface of the moon will be

(A)

(B) M

(C) M 6

(D) zero

**Q12.** Weight

(A) is a vector quantity

(B) of a body in interplanetary space is maximum

(C) increases when the bodies go up

(D) none of these

**Q13.** The value of g near the earth's surface is

(A) 8.9 m/s^{2}

(B) 8.9 m/s

(C) 9.8 m/s^{2}

(D) 9.8 m/s

**Q14.** A geostationary satellite

(A) moves faster than the near earth satellite

(B) has a time period less that of a near earth satellite

(C) revolves about the polar axis

(D) is stationary in space

**Q15.** The force of gravitation between two bodies depend upon

(A) their separation

(B) gravitational constant

(C) product of their masses

(D) all of these

**Q16.** When an object is thrown up, the force of gravity

(A) acts in the direction of the motion

(B) acts in the opposite direction of the motion

(C) remains constant as the body moves up

(D) increases as the body moves up

**Q17.** The force of gravitation exists

(A) everywhere in the universe

(B) at the surface of the earth only

(C) inside the earth only

(D) at the surface of the moon only

**Q18.** 1 kg wt is equal to

(A) 9.8 N

(B) 980N

(C) 98 N

(D) none of these

**Q19.** 1 kg wt is equal to

(A) 980 dynes

(B) 9.80 dynes

(C) 98 dynes

(D) none of these

**Q20.** The value of G does not depend on

(A) nature of the interacting bodies

(B) size of the interacting bodies

(C) mass of the interacting bodies

(D) all of these

**Match the Column**

**Q1.** **Cloumn-I Column-II**

(A) Attraction between two planets (p) gravity

(B) Attraction between a body and a planet (q) weightlessness

(C) Free fall (r) gravitational force

(D) Weight (s) gravitation

**Q2. Cloumn-I Column-II**

(A) A body falling freely (p) displacement

(B) Distance with direction (q) velocity

(C) Speed with direction (r) acceleration

(D) Rate of change of velocity (s) uniformly accelerated motion

**Q3.** **Cloumn-I Column-II**

(A) Wide straps of school bags (p) motion of planets around the sun.

(B) Archimedes principle (q) weight of an object

(C) Gravitational force (r) submarine

(D) Upthrust (s) balloons

(t) to reduce pressure

**Q4.** **Cloumn-I Column-II**

(A) Loss in weight (p) gravitational force

(B) Motion of moon around the earth (q) upthrust

(C) Airship (r) increasing pressure

(D) Sharp tip of a needle (s) archimedes principle

(t) centripetal force

**True & False**

**1.** The force of attraction between two bodies is called gravity.

**2.** The value of G depends upon the mass of two objects.

**3.** If a spring balance, holding a heavy object is released, it will read zero weight.

**4.** The value of G is high if the radius of the body is more and less if radius is less.

**5.** The centre of mass and centre of gravity for a small body lie at the same point.

**6.** The gravitational force between two bodies changes if a material body is placed between them.

**7.** The acceleration of a body thrown up is numerically the same as the acceleration of a downward falling body but opposite is sign.

**8.** The value of g is zero at the centre of the earth.

**9.** The inertia of an object depends upon its mass.

**10.** All objects attract each other along the line joining their centre of mass.

**11.** Acceleration due to gravity, g = , where symbols have their usual meanings.

**12.** Relative density has no unit.

**13.** Archimedes' principle does not apply to gases.

**14.** Any solid will sink in water if its relative density is less than unity.

**Fill in the Blanks**

**1.** .................... is the force of attraction between any two bodies in the universe.

**2.** .................... is the force of attraction between a body and a planet.

**3.** Acceleration due to gravity .................... with height from the surface of the earth.

**4.** Acceleration due to gravity .................... with depth from the surface of the earth.

**5.** Acceleration due to gravity is a maximum at the ....................

**6.** .................... of a body changes from place to place but its .................... remains constant.

**7.** The upward force experienced by a body immersed partially or fully in a fluid is called ....................

**8.** Density of a substance is defined as the ratio of the mass of a body to its ....................

**9.** Relative density of a substance is defined as the ratio of the density of the substance to the density of water at ....................

**10.** In cgs the relative density of a substance is .................... equal to its density in gcm^{3}.

**11.** The combined weight of the sinker and cork is .................... than the weight of the sinker alone.

**12.** For a body to float the density of the floating object should be .................... than or equal to the density of the liquid in which it is to float.

**13.** The centre of ..........is a point, where the total upthrust, due to fluid displaced by part ...........of body acts.

**14.** A fish .................... by squeezing out air, from its ....................

**15.** When a body is partly or wholly immersed in a .................... , it experience an ....................

**16.** An iceberg floats with .................... of its volume below ....................

**17.** The unit of upthrust in SI is ....................

**18.** Lactometer is used to measure .................... of ....................

**19.** A hydrometer sinks .................... in water than in pure milk.

**20.** The density of hot air is .................... that of cold air.

**Answers**

**Match the Column**

1. (A ® s), (B ® p), (C ® q), (D ® r) 2. (A ® s), (B ® p), (C ® q), (D ® r)

3. (A ® t), (B ® r, s), (C ® p, q) (D ® r, s) 4. (A ® q, s), (B ® p,t), (C ® q, s), (D ® r)

**True & False**

1. False 2. False 3. True 4. True 5. True 6. False 7. True

8. True 9. True 10. True 11. True 12. True 13. False 14. False

**Fill in the Blanks**

1. gravitation 2. gravity 3. decreases 4. decreases

5. poles 6. weight, mass 7. buoyant force 8. volume

9. 4°C 10. numerically 11. less 12. less

13. buoyancy, the immersed 14. dives, floating tube 15. fluid, upthrust 16. 11/12, water

17. Newton 18. purity, milk 19. more 20. less than

Offer running on EduRev: __Apply code STAYHOME200__ to get INR 200 off on our premium plan EduRev Infinity!

90 videos|225 docs|135 tests