Question 39:
(a) State the law of conservation of momentum.
(b) Discuss the conservation of momentum in each of the following cases :
(i) a rocket taking off from
(ii) flying of a jet
Solution :
a) According to the law of conservation of momentum: When two (or more) bodies act upon one another, their total momentum remains constant (or conserved) provided no external forces are acting. It means that when one body gains momentum, then some other body loses an equal amount of momentum i.e. momentum is neither created nor destroyed.
b)
a.
b. Flying of
In jet
Question 40:
(a) If a balloon filled with air and its mouth untied/ is released with its mouth in the downward direction, it
moves upwards.
(b) An unloaded truck weighing 2000 kg has a maximum acceleration of 0.5 m/s^{2}. What is the maximum acceleration when it is carrying a load of 2000
Solution :
a) If of a balloon filled with compressed air and its mouth untied is released with its mouth in the downward direction, the balloon moves in the upward direction because the air present in the balloon rushes out in the downward direction. The equal and opposite reaction of downward going air pushes the balloon upwards.
b) Mass of the unloaded truck, m_{1}= 2000 kg
Acceleration a_{1} = 0.5 m/s^{2}
Mass of loaded truck, m_{2} = 2000+ 2000 = 4000 kg
Acceleration a_{2}
m_{1} x a_{1} = m_{2} x a_{2}
_{}
Lakhmir Singh Physics Class 9 Solutions Page No:77
Question 51:
Why are car seatbelts designed to stretch
Solution :
Car seatbelts are somewhat stretchable so as to increase the time taken by the passengers to fall forward. Due to this, the rate of change of momentum of passengers is reduced and
Question 52:
The troops (soldiers) equipped to be dropped by parachutes from an aircraft are called paratroopers. Why do paratroopers roll on
Solution :
The paratroopers roll on landing to increase the time taken to reduce the momentum of their body. Thus, the rate of change of momentum is reduced and hence less force is exerted on their legs and they do not get hurt.
Question 53:
Why would an aircraft be unable to fly on the
Solution :
An aircraft needs air because air moving under the wings of aircraft is strong enough to hold it up and
Question 54:
Explain why it is possible for a small animal to fall from a considerable height without any injury being caused when it reaches the ground.
Solution :
It is possible for a small animal to fall from a considerable height without being injured because a small animal has
Question 55:
A boy of mass 50 kg running at 5 m/s jumps on to a 20 kg trolley
Solution :
Mass of the boy, m_{1}= 50 kg
Speed of boy, u_{1} = 5 m/s
Mass of trolley m_{2} = 20 kg
Speed of trolley u_{2} =1.5 m/s
Combined mass of boy and trolley, m = 20+ 50 = 70 kg
Combined velocity v
Acc. to the law of conservation of momentum
m_{1}u_{1} + m_{2}u_{2} = mv
50 x 5 + 20 x 1.5 = 70 x v
Question 56:
A girl of mass 50 kg jumps out of a rowing boat of mass 300 kg on to the bank, with a horizontal velocity of 3 m/s. With what velocity does the boat begin to move
Solution :
Mass of the boat m_{b} = 300 kg
Velocity of boat v_{b}
Mass of girl m_{g} = 50 kg
Velocity of girl v_{g} = 3 m/s
Acc. to the law of conservation of momentum
m_{b}v_{b}=m_{g}v_{g}
300 x v_{b} = 50 x 3
Question 57:
A truck of mass 500 kg moving at 4 m/s collides with another truck of mass 1500 kg moving in the same direction at 2 m/s. What is their common velocity just after the collision if they move off
Solution :
Mass of first truck, m_{1}= 500 kg
Speed of first truck, u_{1} = 4 m/s
Mass of second truck, m_{2} = 1500 kg
Speed of second truck, u_{2} =2 m/s
Combined mass of both trucks, m = 1500 + 500 = 2000 kg
Combined velocity v
Acc. to the law of conservation of momentum
m_{1}u_{1} + m_{2}u_{2} = mv
500 x 4 + 1500 x 2 = 2000 x v
Question 58:
A ball X of mass 1 kg
Solution :
Mass of the ball x, m_{1}= 1 kg
Speed of ball x, u_{1} = 2 m/s
Mass of ball y, m_{2} = 1 kg
Speed of ball y, u_{2} =0 m/s (at rest)
Velocity of ball x after collision, v_{1} = 0 m/s
Velocity of ball y after collision, v_{2}
Acc. to the law of conservation of momentum
m_{1}u_{1} + m_{2}u_{2} = m_{1}v_{1} + m_{2}v_{2}
1 x 2 + 1 x 0 = 1 x 0 + 1 x v_{2}
Question 59:
A heavy car A of mass 2000 kg
Solution :
Mass of car A, m_{1}= 2000 kg
Speed of car A, v_{1} = 10 m/s
Mass of car B, m_{2} = 500 kg
Speed of car B, v_{2}
Acc to law of conservation of momentum
m_{1}v_{1} = m_{2}v_{2}
2000 x 10 = 500 x v_{2}
Question 60:
A man wearing a bulletproof vest stands still on roller skates. The total mass is 80 kg. A bullet of mass 20 grams is fired at 400 m/s. It is stopped by the vest and falls to the ground. What is then the velocity of the
Solution :
Mass of the man, m_{1}= 80 kg
Speed of man, v_{1}
Mass of bullet m_{2} = 20 g = 0.02 kg
Speed of bullet v_{2 }= 400 m/s
Acc to law of conservation of momentum
m_{1}v_{1} = m_{2}v_{2}
80 x v_{1} = 0.02 x 400
1 videos228 docs21 tests

1. What is the force and law of motion? 
2. What are the three laws of motion? 
3. How does force affect motion? 
4. What are some examples of forces in everyday life? 
5. How are the force and motion related? 
1 videos228 docs21 tests


Explore Courses for Class 9 exam
