Federal Board, FBISE Class 9 Physics Notes of Chapter 3 Exercise are given below. All the Chapter 3 exercise questions are solved that are present in reduced syllabus.

## Chapter 3 Exercise

### Short questions

#### 3.2 Define the following terms:

Ans.    (i)         Inertia
Inertia is the property of the body due to which it resists any change in its state of rest or uniform motion. Inertia depends on the mass of the body. The greater the mass of a body, the greater is its inertia.

(ii)        Momentum
It is the quantity of motion it possesses due to its mass and velocity. The momentum P of a body is given by the product of its mass m and velocity v. Thus
P = mv
Momentum is a vector quantity. Its SI unit is kgms-1.

(iii)       Force
Force is that agent which moves or tends to move, stops, or tends to stop the motion of a body. The force can also change the direction of motion of a body. It is a vector quantity and its unit is newton (N). Mathematically                                                                                    F = ma

(iv)       Force of Friction
The force that opposes the motion of one surface over another surface or through any medium is called friction. It is caused by the roughness of two surfaces.

(v)        Centripetal Force
Centripetal force is a force that keeps a body to move in a circle. Centripetal force always acts perpendicular to the motion of the body.  Mathematically,
Fc = mv2/r

#### 3.3 What is the difference between:

Ans.    (i)         Mass and weight

(ii)        Action and reaction
Action and Reaction are always equal and opposite, but they do not act on the same body. They act on two different bodies.

(iii)       Sliding friction and rolling friction
A force between the sliding objects which opposes the relative motion between them is called sliding friction.
Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than sliding friction.

#### 3.4 What is the law of Inertia?

Ans.    The first law of motion is directly related to inertia. Therefore first law of motion is also known as the law of inertia.

#### 3.5 Why is it dangerous to travel on the roof of a bus?

Ans:    When a bus suddenly takes a turn, stops, or suddenly speeds up, the passengers on the roof of the bus cannot maintain their balance due to inertia. Therefore they may fall on either side and may get hurt.

#### 3.6 Why does a passenger move outward when a bus takes a turn?

Ans:    It is due to inertia that the passengers want to continue their motion in a straight line and thus fall outward.

#### 3.7 How can you relate a force with the change of momentum of a body?

Ans.    Consider a body of mass m moving with initial velocity vi. Let a force F acts on the body which produces an acceleration ‘a’ in it. This changes the velocity of the body. Let its final velocity after time becomes vf. If Pi and Pf be the initial momentum and final momentum of the body related to initial and final velocities respectively, then
Pi = m vi
And                 Pf = m vf
Change in momentum = Final momentum – initial momentum
Pf – Pi    =   m vf   – m vi
Thus the rate of change of momentum is given by:

Thus it is proved that rate of change of momentum is equal to applied force.

#### 3.8 What will be tension in a rope that is pulled from its ends by two opposite forces 100 N each?

Ans.    As the two forces of 100 N each are opposite, therefore the tension in the rope will be equal to the sum of both forces i.e.                                        T = 100+100 = 200 N

#### 3.9 Action and reaction are always equal and opposite. Then how does a body move?

Ans. Action and reaction forces are equal and opposite, but they are not balanced forces because they act on different objects so they don’t cancel out. Hence, the body moves due to the resultant force of action and reaction forces.

#### 3.10 A horse pulls a cart. If action and reaction are equal and opposite then how does the cart move?

Ans: When a horse pulls a cart, the action is on the earth because the horse pushes the earth in the backward direction with its feet. The reaction of earth moves the cart and horse forward, because net force on the horse and cart is only in the forward direction.

#### 3.11 What is the law of conservation of momentum?

Ans.    Law of Conservation of Momentum
Law of Conservation of momentum states that “The momentum of an isolated system of two or
more than two interacting bodies remain constant.”

Consider an isolated system of two spheres of masses ‘m1’ and ‘m2’ moving with velocities ‘u1’ and ‘u2’ respectively in the same direction. If u1> u2, The Two spheres will collide with each other. Let their velocities become ‘V1’ and ‘V2’ respectively after the collision. Thus the

Total momentum before collision = m1u1 + m2u2

Total momentum after collision = m1v2 + m2v2

According to the law of conservation of momentum

m1u1 + m2u2 = m1v1 + m2v2

#### 3.12 Why is the law of conservation of momentum important?

Ans.  The law of conservation of momentum is very important because rockets and jet engines also work on the same principle. In these machines, hot gases produced by burning of fuel rush out with large momentum. The machines gain an equal and opposite momentum. This enables them to move with very high velocities.

#### 3.13 When a gun is fired, it recoils. Why?

Ans:  Before firing, the total momentum of the gun and bullet is zero. After firing the bullet moves forward with great speed. To keep the total momentum zero after firing, the gun recoils (moves backward).

#### 3.14 Describe two situations in which force of friction is needed.

Ans.     Friction is needed in the following situations:
1.         Between our shoes and the floor stop us from slipping.
2.         Between tyres and the road stop cars from skidding.
3.         Between the brakes and wheel help bikes and cars to slow down.

#### 3.15 How does oiling the moving parts of a machine lower friction?

Ans.    Friction exists due to interlocking of ups and downs on the surfaces. While oiling various parts of the machine, downs are filled by oil which makes it smooth and slippery. They can easily move against each other, and friction between them is reduced.

#### 3.16 Describe ways to reduce friction.

Ans:    Methods to Reduce Friction:
1.  Friction can be reduced by making the surfaces in contact smooth and highly polished.
2.  By lubricating the machine parts or by putting a layer of grease or oil between them the friction will be reduced.
3.  Sliding friction is converted into rolling friction by using wheels and ball bearings.
4.  The front of the fast-moving objects like cars and planes is made oblong and pointed shaped to reduce the air friction.

#### 3.17 Why rolling friction is less than sliding friction?

Ans.    In rolling friction interlocking between two surfaces is not needed to break because two surfaces have minimum area of contact while in sliding friction these ups and downs are to be broken due to which large force of friction is produced. The rolling friction is about 100 times less than sliding friction.

#### 3.18     What you know about the following:

(i) Tension in a string

Excluded from Syllabus for Annual Exam 2021

(ii)  Limiting force of friction
The maximum value of friction is known as the force of limiting friction (FS). It depends on the normal reaction (pressing force) between the two surfaces in contact.
Mathematically                                   FS = m R

(iii)  Braking force

Excluded from Syllabus for Annual Exam 2021

(iv)  Skidding of vehicles

Excluded from Syllabus for Annual Exam 2021

(v)   Seatbelts

Excluded from Syllabus for Annual Exam 2021

Excluded from Syllabus for Annual Exam 2021

(vii)   Cream separator

Excluded from Syllabus for Annual Exam 2021

#### 3.19 What would happen if all friction suddenly disappears?

Ans: If all the friction suddenly disappears, it may cause a lot of damages and accidents. Even walking on the ground would be impossible. In short everything would be out of order and whole system on earth would collapse.

#### 3.20  Why the spinner of a washing machine is made to spin at a very high speed?

Excluded from Syllabus for Annual Exam 2021

MCQs of Chapter 3 Exercise are given separately, in another post.

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