Solution of Force and Law of Motion (Page No - 74) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 1:
Which physical quantity corresponds to the rate of change of momentum ?
Solution :
Force corresponds to the rate of change of momentum.

Question 2:
State the relation between the momentum of a body and the force acting on it.
Solution :
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts.

Question 3:
What is tire unit of force ?
Solution :
The SI unit of force is newton(N)

Question 4:
Define one newton force.
Solution :
A newton force is defined as that force which when acting on a body of mass of 1 kg produces an acceleration of 1m/s² in it.

Question 5:
What is the relationship between force and acceleration ?
Solution :
Force acting on a body is directly proportional to the acceleration produced in the body.

F∝a

Question 6:
If the mass of a body and the force acting on it are both doubled, what happens to the acceleration ? 

Solution : 

Acceleration remains same since

Question 7:
Name the physical quantity whose unit is ‘newton’.
Solution :
Newton is the SI unit of force.

Question 8:
Which physical principle is involved in the working of a jet aeroplane ?
Solution :
Jet airplanes work on the principle of conservation of momentum.

Question 9:
Name the principle on which a rocket works.
Solution :
Rockets work on the principle of conservation of momentum.

Question 10:
Is the following statement true or false :
A rocket can propel itself in a vacuum.
Solution :
True, because rocket does not require air for obtaining uplift or for burning its fuel.

Question 11:
What is the force which produces an acceleration of 1 m /s²  in a body of mass 1 kg ?
Solution :
Mass, m = 1 kg
Acceleration, a =
1m/s²
Force F = m x a = 1
x 1= 1N

Question 12:
Find the acceleration produced by a force of 5 N acting on a mass of 10 kg.
Solution :
Force F =5 N
Mass m = 10 kg

Question 13:
A girl weighing 25 kg stands on the tloor. She exerts a downward force of 250 N on the floor. What force does the floor exert on her ?
Solution :
The force exerted by the floor on her = the downward force exerted by the girl =250 N
This is due to Newton’s third law of motion which states that to every action there is an equal and opposite reaction.

Question 14:
Name the physical quantity which makes it easier to accelerate a small car than a large car.
Solution :
Less mass of the small car makes it easier to accelerate a small car than a large car because acceleration is inversely
proportional to mass of the car

Question 15:
Fill in the following blanks with suitable words :
(a) To every action, there is an………………….. and…………………… reaction
(b)  Momentum is a…………….. Its emit is……………………
(c)  Newton’s second law of motion can be written as Force = mass x …………………………… or Force = ……………….. of change of……………….
(d)  Forces in a Newton’s third law pair have equal………………… but act in opposite………………..
(e)  In collisions and explosions, the total ……………… remains constant, provided that no external……………………..
Solution :
a) Equal; opposite
b) Vector; kg.m/s
c) Acceleration; rate; momentum
d) Magnitude; directions
e) Momentum; force

Question 16:
Explain the meaning of the following equation :
F = m x a
where symbols have their usual meanings
Solution :
Force is directly proportional to the product of ‘mass’ of the body and the ‘acceleration’ produced in the body by the action of force.
F = m x a
where F is the force applied on the body
m is the mass of the body
a is the acceleration produced in the body

Question 17:
To take the boat away from the bank of a river, the boatman pushes the bank with an oar. Why ?
Solution :
To take boat away from the bank of a river, the boatman pushes the bank with the oar. The bank exerts an equal and opposite force on the boat which makes the boat move forward away from the bank.

Question 18:
Why does a gunman get a jerk on firing a bullet ?

Solution :
Gunman gets a jerk on firing a bullet because when a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backwards but due to high mass of the gun, it moves only a little distance backwards giving a jerk to the gunman.

Question 19:
If action is always equal to reaction, explain why a cart pulled by a horse can be moved.
Solution :
To make the cart move, the horse bends forward and pushes the ground with its feet. When the forward reaction to the backward push of the horse on the ground is greater than the opposing frictional forces of the wheels, the cart moves.