Class 9 Physics: Force and Laws of Motion – Notes

Class 9 Science: Force and Laws of Motion – Notes

Motion describes change of position with time

• Displacement (s), velocity (v), acceleration (a) are descriptors
• Previous chapters described how objects move but not why

What is Force?

• A push or pull acting on a body which tends to change its state of rest or of motion is called a force. Denoted by F.

e.g. Kicking a ball, pulling a box etc.
Or,
Force is an interaction that changes or tries to change the state of motion or the shape of an object.
• Vector quantity: Requires both magnitude and direction
• SI unit: Newton (N)
• Force explains the cause behind changes in motion.
• Force itself is not visible. We know a force is present because of its effects — like movement, stopping, or change in shape.

1 N = force required to produce acceleration of 1 m/s² in 1 kg mass

• 1 kgf = 9.8 N or approx. 10 N
• Force is required to change the state of motion of an object — either to start, stop, speed up, slow down, or change direction.
• Once an object is already in motion (especially uniform motion), it does not require a continuous cause or force to keep moving.
• Historically, people thought rest was the natural state of objects until Galileo and Newton showed that motion doesn’t need a continuous force, only a force to change motion.

Effect of Force:

• Change speed of an object
• Change direction of motion
• Bring an object to rest or move it
• Change shape or size
Force is required to change motion, not to maintain it.
Spring Balance: Device to measure force

Balanced vs Unbalanced Forces

Balanced Forces:

When the resultant of two or more forces acting on a body is zero, the forces are said to be the balanced forces.
• Resultant force = 0
• Do not change state of motion
• Can change shape or size
Examples: Two people pushing box with equal force, Book on table

Unbalanced Forces:

When the resultant of two or more forces acting on a body is not zero, the forces are said to be the unbalanced forces.
• Resultant force ≠ 0
• Produce acceleration
• Cause change in motion
Examples: One person pushing harder, ball thrown, braking car
Force affects change in velocity, not velocity itself

Newton’s First Law of Motion

It States that:
“An object remains in a state of rest or uniform motion in a straight line unless compelled to change that state by an external unbalanced force.”
• Force changes natural state of motion
• Motion does not require continuous force
• Known as Law of Inertia
  The First Law says that an object does not change its state on its own. It resists any change in its state of motion or rest — only an unbalanced force can change it. Therefore, the First Law of Motion is also known as the Law of Inertia.
• Rest and uniform motion = zero acceleration
"Internal forces can't change system's motion."
Aristotle’s Fallacy:
Aristotle believed that force is needed to keep an object in motion.
But Galileo and Newton proved this wrong. Established that no force = no acceleration (not no motion)

Inertia

• Definition: Tendency of object to resist change in motion
• More mass → More inertia
• Universal property : Every object with mass exhibits inertia
🔁 Frame Independence: Inertia behaves same in all inertial frames.

📌 That means:

Whether you’re sitting still on the ground or you’re inside a train moving at constant speed

🔸 In both frames, an object at rest stays at rest,

🔸 An object in motion keeps moving unless a force acts on it.

👉 The behaviour of inertia doesn’t change, even though the reference point (frame) has changed.

Types of inertia:

1. Inertia of Rest

It is the tendency of an object at rest to remain at rest unless an external force is applied.

✅ Example: A book on a table stays still until someone pushes it.

2. Inertia of Motion

It is the tendency of a moving object to keep moving with the same speed and direction unless acted upon by an external force.

✅ Example: Passengers in a moving bus fall forward when the bus stops suddenly.

3. Inertia of Direction

It is the tendency of an object to keep moving in the same direction, resisting any change in direction.

✅ Example: When a car turns sharply, passengers tend to fall sideways (in the original direction of motion).

Momentum

• Definition: Momentum is the product of the mass and velocity of a moving object. It is a vector quantity and is always in the direction of the object’s motion.
Momentum (p) = m × v
Units: kg⋅m/s or N⋅s

Rate of Change of Momentum

The rate of change of momentum means how quickly the momentum of an object changes over time.

Or,

The rate of change of momentum is the change in momentum per unit time.

• Initial momentum = m × u
• Final momentum = m × v
• Change in momentum = m(v - u)
• Rate of change of momentum = m(v - u)/t
🧠 A cricketer pulls back his hands while catching a fast ball to increase the time taken to stop the ball.
This reduces the rate of change of momentum, and as a result, the force acting on the hands becomes less.
Hence, it helps in preventing injury.

Newton’s Second Law of Motion

“ The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.”
• Formula: F = ma
• Force causes acceleration
• No force = No acceleration
• Direction of force = Direction of acceleration
• It provides a quantitative definition of Force F = ma. (i.e. Makes force calculation possible and precise)
Heavier mass = less acceleration for same force

Force Formulas

• F = ma
• F = (Change in momentum) / Time
• F = m(v - u)/t

Impulse

Impulse is the effect of a force acting on an object for a short time.

It is equal to the product of force and the time for which it acts.

It is a vector quantity.

• Impulse = Force × Time
• Also, Impulse = Δp = m(v - u)
• Unit: N·s = kg·m/s
🔁 Impulse explains how a small force acting for a long time or a large force acting for a short time can produce the same effect.
• Cricketer catching ball
• Hammer hitting nail

Newton’s Third Law of Motion

“To every action there is equal and opposite reaction.”
F_AB = −F_BA
• Act on different bodies
• Equal in magnitude, opposite in direction
Action-reaction don't cancel as they act on different objects

Action-Reaction Examples:

Situation	Action Force	Reaction Force
Walking	Foot pushes ground backward	Ground pushes foot forward
Swimming	Hand pushes water backward	Water pushes hand forward
Gun firing	Gun pushes bullet forward	Bullet pushes gun backward
Rocket launch	Engine pushes gas downward	Gas pushes rocket upward

Momentum Conservation

“Total momentum of system is conserved if no external force acts.”
• Closed system: total momentum before = after

Formula:

Let:

Object A has mass m₁ and velocity u₁ before collision.

Object B has mass m₂ and velocity u₂ before collision.

Their velocities after collision are v₁ and v₂.

Total initial momentum = m₁u₁ + m₂u₂
Total final momentum = m₁v₁ + m₂v₂

So,

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂