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Question-1

Which of following have more inertia : (a) a rubber ball and a stone of same size ? (b) a bicycle and train ? (c) five rupee coin and a one rupee coin.

Solution:

(a) stone (b) train and (c) five rupee coin.
 

Question-2

In the following example, try to identify the number of times the velocity of ball changes:

"A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards the player of his own team".

Also identify the agent supplying the force in each case.

Solution:
The velocity of ball changes three times.

First time, the velocity changes when the football player of one team kicks the ball.

Second time, the velocity changes when another player of same team kicks the football.

Third time, the velocity changes when the goalkeeper of the opposite team kicks the football. Bold letters has indicated the force supplied by the agents in each case.

Question-3

Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Solution:
Before shaking the branches, leaves are at rest. When branches are shaken
they acquire inertia of motion, while the leaves tend to remain at rest due to inertia of rest. As a result leaves get detached from the branches and fall down.

Question-4

Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Solution:
When a moving bus brakes to a stop, the lower part of our body in contact with
the bus comes to rest while the upper part of our body tends to keep moving due to inertia of motion. Hence we fall forwards. When the bus accelerates from rest, the lower part of our body comes into motion along with the bus while the upper part of tends to remain at rest due to inertia of rest. Hence we fall backwards

Question-5

If action is always equal to the reaction, explain how a horse can pull a cart.

Solution:
The horse pulls the cart with a force (action) in the forward direction. The cart also pulls the horse with an equal force (reaction) in the backward direction. The two forces get balanced. While pulling the cart, the horse also applies enough force to overcome friction and bends forward to push the ground with its feet in the backward direction. The forward reaction of the earth makes it move in the forward direction overcoming the friction between the wheels and the road.

Question-6

Explain, why is it difficult for a fireman to hold a hose, which ejects large amount of water at a high velocity.

Solution:
The hose has a tendency to go backward due to the backward reaction of water rushing through it in the forward direction. It is ejected with a large forward force (action) and due to reaction; the fireman experiences a large backward force. So, he feels difficulty in holding the hose.

Question-7

From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35ms-1. Calculate the initial recoil velocity of the rifle.

Solution:
Mass of bullet, m1=50 g=0.05 kg

Mass of rifle, m2=4kg

Initial velocity of bullet, u1=0

Initial velocity of rifle, u2=0

Final velocity of bullet v1=35ms-1

Final velocity of rifle v2= ?

According to the law of conservation of momentum,

Total momentum after the fire= Total momentum before the fire

   m1v1 + m2v2 = m1u1 + m2u2

0.05
× 35 + 4v2= 0 + 0 or v2= - = -0.44 ms-1. 
The negative sign indicates that the direction in which the rifle would recoil is opposite to that of the bullet.

Question-8

Two objects of masses 100 g and 200g are moving in along the same line and direction with velocities of 2 ms-1 and 1 ms-1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67ms-1. Determine the velocity of the second object?

Solution:
Here, m1=100g =0.1 kg, m2=200 g = 0.2 kg, u1=2 ms-1, u2=1 ms-1,v1

v1 =1.67 ms-1, v2=?

According to the law of conservation of momentum,

m1u1+m2u2 = m1v1+m2v2

0.1
× 2+0.2× 1=0.1× 1.67+0.2 v2

v2== 1.165ms
-1
 

Question-9

When a carpet is beaten with a stick, dust comes out. Explain why?

Solution:
Initially, both the carpet and the dust are at rest. When the carpet is beaten with a stick, the carpet is set into motion. Due to inertia of rest, the dust particles tend to remain at rest. As a result, the dust particles come out.

Question-10

What happens when you shake a wet piece of cloth? Explain your observation.

Solution:
When we shake a wet piece of cloth, it dries very quickly, because shaking the wet piece of cloth removes the air, which, when in contact with water gets saturated with moisture and is replaced by a fresh quantity of dry air. This increases the rate of evaporation.

Question-11

A javelin throw is marked foul if the athlete crosses over the line marked for the throw. Explain why athletes often fail to stop themselves before that line?

Solution:
Due to inertia of motion athletes fail to stop themselves before the line marked for the throw.

Question-12

Two similar trucks are moving with the same velocity on a road. One of them is loaded while the other one is empty. Which of the two will require a larger force to stop it?

Solution:
The truck, which is loaded, will require a large force to stop it.

Question-13

Hit on a wall and a piece of sponge with nearly equal force. Did you succeed in doing so?

Solution:
We did not succeed in doing so.

Question-14

If action is always equal to the reaction, explain why a cart pulled by a horse can be moved.

Solution:
While pulling the cart, the horse pushes the ground backwards with its foot by a force. As a reaction, the ground exerts force on the horse equal and opposite to the force applied on it.

Question-15

How much momentum will a dumb-bell of mass 10 kg transfer to the floor of it falls from a height of 0.8 m? Take its downward acceleration to be 10 m/s2.

Solution:
Mass of the dumb-bell = 10 kg
Height, h = 0.8 m
Acceleration due to gravity g = 10 m/s2
Initial velocity u = 0
We know that v2 – u2 = 2
× 10 × 0.8
                           v2 = 16
                            v = 4 m/s
Momentum, p = mv
                    = 10
× 4
                    = 40 kg m/s.

Question-16

What would be the force required to stop a car of mass 1000 kg and a loaded truck of mass 10,000 kg in 2s if they were moving with same velocity of 5 m/s.

Solution:
Initial velocity u = 5 m/s,
Final velocity v = 0
Time taken t = 2s
We know that
v = u + at
0 = 5 + a
× 2
a = -m/s2
Force required to stop the car F = m
× a
                                            = 1000
× m/s2
                                            = 2500 N

(ii) Force required to stop a loaded truck F = m
× a
                                                           = 10,000 kg
×
                                                           = 25,000 N
.

Question-17

Why is it difficult to balance our body when we accidentally step on the peel of a banana?

Solution:
It is difficult to balance our body when we accidentally step on a peel of banana because friction is reduced by the smoothness of the banana skin.

Question-18

Explain why the sole of a shoe worn out.

Solution:
This is because when we walk on the road, there is friction between the soles of our shoes and the surface of road. Due to this the soles our shoes wear out.

Question-19

Quick sand consists of very smooth and tiny grains of sand. What could happen if a man or a heavy animal steps over a patch of such sand in a desert?

Solution:
When a man or heavy animal steps over a patch of sand in a desert, the force of weight of man or animal falls on a smaller area of the ground and produces more pressure on the ground. Therefore, sand would not consist of very smooth and tiny grains of sand.

Question-20

In oil tankers some space is left at the top while filling them. Explain, why?

Solution:
The air space at the top of the oil tankers creates a low pressure inside the tube and a higher pressure outside the tank. Since air flows from higher pressure to lower pressure, it keeps the lid of the tank closed.

Question-21

Why it is advised to tie the luggage with a rope on the roof of buses?

Solution:
When wind blows with very high speed over the topside of the roof, the pressure of air over the roof (P2) becomes low. But below the roof, the wind is not blowing. Therefore, pressure of air under the roof (P1) is very large i.e. 
P1 > P2. This difference of pressure provides a vertical lift to the luggage. Therefore it is advised to tie the luggage with a rope on the roof of buses.

Question-22

A cricket ball of mass 70 g moving with a velocity of 0.5 m/s is stopped by a player in 0.5s. What is the force applied by the player to stop the ball?

Solution:
Mass of cricket ball = 70 kg

                               = kg

Initial velocity u = 0.5 m/s

Time taken t = 0.5 s

Final velocity v = 0

We know that

v = u + at

0 = 0.5 + a
× 0.5

a = m/s2

F = m
× a

  = 0.07 N
.

Question-23

A car of mass 1000 kg moving with a velocity of 45 km/h collides with a tree and comes to a stop in 5s. What will be the force exerted by the car on the tree?

Solution:
Mass of the car, m = 1000 kg

Initial velocity u = 40 km/h = m/s

Time taken t = 5s

Final velocity v = 0

We know that

v = u + at

0 =


a = m/s2

Force = m
× a = 1000 ×

                    = = 2222.2 N.

Question-24

What will be acceleration of a body of mass 5 kg if a force of 200 N is applied on it?

Solution:
Mass of the body, m = 5 kg

Force F = 200 N


Acceleration, a =

                        = 40 m/s2
.

Question-25

A bullet of mass 10 g is fired with a rifle. The bullet takes 0.003 s to move through its barrel and leaves it with a velocity of 300 m/s. What is the force exerted on the bullet by the rifle?

Solution:
Mass of the bullet = 10 g = 100 × 10-3 = 0.01 kg
Time taken t = 0.003 s
Initial velocity u = 0
Final velocity v = 300 m/s
We know that,

v = u + at
300 = 0 + a
× 0.003

a = m/s2

Force F = m
× a = 0.01 × 105 = 103 N.

Question-26

If someone jumps to the shore from a boat, the boat moves in opposite direction. Explain why.

Solution:
A person requires certain momentum (i.e., mass × velocity) to jump out of a boat. To gain force in the forward direction, the person applies force in the forward direction. The reaction of this force acts on the boat but in the opposite direction. As a result, when the person moves in the forward direction, the boat moves backward.

Question-27

Explain, why a glass pane of a window is shattered when a flying pebble hits it?

Solution:
When a flying pebble hits the glass pane, a much larger area of the glass pane gets enough time to share the velocity of the stone. As a result the glass is shattered.

Question-28

Explain, why it is difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

Solution:
When firemen direct a powerful stream of water on the fire from a hose pipe, they have to hold the hose pipe strongly so that it does not move backward. This backward movement of the hose pipe is due to the backward reaction of the water rushing through the pipe in the forward direction at a great speed.

Question-29

A large truck and a car, both moving with a velocity of magnitude v, have a head-on collision and both of them come to halt after that. If the collision lasts for 10s:

(a) Which vehicle experiences the greater force of impact?
         
(b) Which vehicle experiences the greater momentum change?
         
(c) Which vehicle experiences the greater acceleration?
         
(d) why is the car likely to suffer more damage than the truck?

Solution:
(a) Car experiences the greater force of impact.

(b) Truck experiences the greater momentum change.

(c) Car experiences the greater acceleration.

(d) Car is likely to suffer more damage than the truck because greater force will be exerted on the car.

Question-30

Amar, Javet and Rita were riding in a bus that was moving with a high velocity on a highway when a bug hit the windshield and its dead body got stuck on the windscreen. Amar and Javed began discussing the situation. Amar suggested that the change in momentum of the bug is much greater than that of the bus. After all, argued Amar, there was no noticeable change in the velocity of the bus compared to the obvious change in the velocity of he bug. Javed opined that since bus was moving with a larger velocity, so it exerted a larger force on the bug, as a result of which it died. Rita disagreed entirely, arguing that both the bug and bus experience the same force and change in their momentum. Who do you agree with? Support your answer.

Solution:
The change in momentum of the bus is very small, hence it is not noticeable.
The change in momentum of bug is greater. Since force is directly proportional to the rate of change of momentum, the bug will experience greater force of impact and hence it dies.

Question-31

Why does a mug full of water feel lighter inside water?

Solution:
When a mug is immersed in water, it experiences an upward buoyant force due to which it feels lighter inside water.

Question-32

The volume of a 500 g sealed tin is 350 cm3. What is the density of the packed tin? Will the packed tin float or sink in water if it has the density 1 g/cm3? What will be the mass of the water displaced by this tin?

Solution:
Mass of sealed tin, m = 500 g
                                = 0.5 kg
Volume of sealed tin, V = 350 cm3

Density of the tin, d = = 1.4 g/cm3

The density of water is 1 g/cm3,
Since the density of water is less than the density of tin, the tin will sink in water.
Weight of the water displaced = 350 cm3.

Question-33

What will be the relative density of the packed tin in the question No. 25?

Solution:
Upthrust (Volume of the water displaced) = d × g
                                                             = 350
× 10-6 × 1000 × 10
                                                             = 3.5 N
Apparent weight = Weight
× Upthrust

                        = (0.5
× 10) – 3.5

                        = 5 – 3.5 = 1.5 N

Relative density of packed tin = = 3.3
.




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