# Chapter 9: Force and Laws of Motion Class 9 Notes Pdf

## Force and Laws of Motion Class 9 Notes

Topics Discussed:

Chapter Overview –

• What is Force?
• Effects of force
• Types of Forces
• Newton’s Laws of Motion
• Newton’s First Law of Motion
• Newton’s Second Law of Motion
• Newton’s Third Law of Motion

### What is Force?

A force is an interaction that brings changes in the motion of a body if unopposed. It can also be defined as a “push” or “pull” on a body.

Some examples of force-

• Kicking a football (push).
• Opening a drawer (pull)

Mathematically, Force = mass × acceleration

• SI Unit of Force is kg.m/s² or Newton (N).

### Effects of Force

1. A Force can move a stationary body.

Example: -upon kicking a football at rest it starts moving

• Flicking a card moves it from the rest
• The engine of a car moves it on the application of force

If a body was at rest and it starts moving then a force must have been applied to it

2. A force can stop a moving body

Example: –   Brakes of cycle stop it by applying force on the wheels of the cycle.

A fielder on the ground stops a moving ball by applying force on it.

3. A force can change the speed of a moving body.

Example: –   Rolling speed decreases due to friction force.

A force can increase and decrease the speed of a moving body.

4. Force can change the direction of a moving body.

Example:-

• A batsman changes the direction of a moving ball by hitting it.
• kicking a football in opposite direction, it’s the direction of motion changes.

5. A force can change the shape and size of a moving body.

Example:- Stretching a rubber band makes it longer.

• On compressing a balloon the shape changes.

When Force is applied to a body in the opposite direction of motion its speed decreases.

### TYPES OF FORCES

Forces are of two types:-

1. Balanced Forces
2. Unbalanced Forces

#### BALANCED FORCES

If the resultant of all the Forces acting on a body is zero, then the forces are said to be Balanced.

Read Also: Chapter 8 – Motion Class 9 Notes

• Balanced forces acting o a body at rest cannot make it move.
• Balanced forces cannot change the speed of a body moving with a uniform (constant) speed in a straight line.
• Balanced forces acting on a body cannot change the direction of motion of the body.
• Balanced forces acting on a body cannot change the state of motion of the body.
• The balanced force acting on a body can change the shape of the body.

Example:- Stretching a rubber band elongates it.

#### UNBALANCED FORCES

If the resultant of all the forces acting on a body is not zero, then the forces are said to be Unbalanced.

All the effects that can be produced by force are the result of Unbalanced forces.

Examples-

• moving a body from rest.
• changing the speed and direction.
• changing the shape of the body.

### Newton’s Laws of Motion

Newton gave three laws that tell about the motion of bodies, called Newton’s Laws of Motion. Let us understand the three laws of motion given by Sir Issac Newton.

#### Newton’s First Law of Motion

It states that a body at rest will remain at rest or a body moving with a constant velocity will keep moving with constant velocity unless it is compelled by an external force.

Examples of Newton’s First Law of Motion-

1. A football at rest will remain at rest unless an external force is applied to it.
2. A book kept on the table will remain at rest unless it is pushed.
3. A cycle would keep moving if we stopped pedaling if there were no frictional forces and air resistance.

#### Inertia

The tendency of a body to remain at rest if it was at rest or keep on moving with a constant velocity or speed if it was moving with a constant velocity or speed, is called Inertia.

• Inertia is a property in which a body resists change in its state of motion or rest.
• The mass of a body is the measure of its Inertia, more mass means more Inertia, and more mass means more Inertia.

Example: A cricket ball has more inertia than a tennis ball because a cricket ball has more mass than a tennis ball.

• Newton’s first law of motion is also known as Galileo’s Law of Motion.

OBSERVATIONS BASED ON INERTIA:

• When blankets are beaten dust particles come out of them.
• Passengers fall backward when a bus suddenly moves.
• When a moving bus suddenly stops passengers are jerked forward.
• Jumping from a moving train can be dangerous.
• Seat belts save us from the sudden movements of a car.

### Concept of Mass

• The mass of a body is defined as the actual content or matter present in a body.
• The SI Unit of mass is kg.
• It is a scalar quantity.

How Mass is different from Weight?

Weight is the force that is applied by the earth.

### Momentum

The momentum of a body is defined as the product of its mass (m) and its velocity (v).

p = m × v

• Momentum gives an idea of the quantity of motion carried by a body.
• Momentum is a vector quantity.
• The unit of Momentum is kg-m/s

#### Newton’s Second Law of Motion

It states “the rate of change of momentum is directly proportional to the unbalanced force applied”.

Force ∝ rate of change of momentum

Force ∝ Δp/Δt

Force = K×(Δp/Δt), where ‘K’ is an arbitrary constant and K=1

So, Force = Δp/Δt

Force = change in momentum/time taken

• Force is a vector quantity.

Define 1 newton of force.

It is the amount of force that will produce an acceleration of 1 m/s² in a body of mass 1 kg.

#### Newton’s Third Law of Motion

It states that if a body exerts a force on another body, the second body exerts an equal amount of force on the first body in the opposite direction.

It can also be stated as “To every action, there is an equal and opposite reaction.”

• The Action and Reaction forces act on different bodies or forces are colinear
• Action and Reaction forces will be on the same line.
• Forces are of the same nature.
• No delay in Action-Reaction.

### Application of Newton’s 3rd Law

1. HELPS IN WALKING

While walking we push the ground backward, and the ground, in turn, pushes us forward.

2. RECOIL OF GUN

While firing a gun, a force in the backward direction is experienced.

3. ROCKET PROPULSION

• Rocket Propulsion is based on Newton’s 3rd Law of Motion.
• The rocket exerts a force on the gases and in turn, the gas exerts a force on the rocket.

### Conservation of Momentum

Law of Conservation of Momentum

The Law of Conservation of Momentum states that the sum of momenta of two bodies before and after the collision will always remain the same if there is no external unbalanced force acting upon the colliding bodies.

Total Momentum before collision = Total Momentum after collision

m1u1+m2u2 = m1v1+m2v2

#### Class 9 Force and Laws of Motion Numericals

1. How much momentum a hammer of 5 kg will transfer to the floor if it is dropped from a height of 90 cm? Take g 10m/s2

2. A body of mass m traveling with a velocity ‘v’ changes its velocity to thrice the initial. Find the change in momentum and the percentage change also.

For more Class 9 Force and Laws of Motion Numericals, you can click the link below

Physics Numerical: Force and Laws of Motion Class 9 Numerical

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