Class 9 Science Chapter 8 Question Answers - Force and Laws of Motion

Q1: State all 3 Newton’s law of motion.
Ans: 

• Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
• Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
• Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Q2: Explain inertia and momentum.
Ans :

• Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position.
• Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the product of the mass and velocity of the body and is denoted by p. p = mv Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms^–1).

Q3: Define force. What are different types forces?
Ans: 

• Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
• Types of forces : Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
• Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
• Frictional force : Force of friction is the force that always opposes the motion of object.

Q4: What is inertia? Explain different types of inertia.
Ans: 

• Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of its inertia. Its S.I. unit is kg.

Types of inertia :

• Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
• Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not applied on it.

Q5: Give example to show the effects of force.
Ans: 
(i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Q6: (a) What is friction?
(b) What are the advantages of friction? or Why is friction necessary?
Ans: 
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :

• The nails and screws hold the wooden boards together due to friction. 
• It is due to the friction between the ground/road and the soles of our shoes that we are able to walk. 
• The friction between the road and the surface of the tyres permits safe driving. 
• W e are able to write on paper because of the friction between the pen/pencil and the paper. That is why, it is very difficult to write on a glazed/waxed paper. 
• The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Q7: (i)  What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?  or  Describe some method for reducing friction.
Ans: The friction rises due to the following factors : 

• Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion. 
• Due to the interlocking of the surface irregularities. 
• Thus, friction is due to the roughness of the two surfaces in contact. 
• The friction between two surfaces can be reduced by following methods : 
  • By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction. 
  • By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Q8: When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans: 
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms² 
From the relation F = m a, we have  
40 = m × 5
m = 40/5 = 8 kg

Q9: An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans: Given,
Acceleration, a = 8 ms^–2 
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion,
s = ut + 1/2 at², one gets  
s = 0 × 1 + 1/2 × 8 × 1² = 4 m
The object travels a distance of 4 m.

Q10: Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans:
Here
u = 0 ms^–1;
v = 30 ms^–1;
t = 10 s;
a = ?
Using v = u + at, we have  
30 = 0 + a (10) a
= 3 ms–2
Now
F = ma
= 1,500 × 3 or F = 4,500 N