Notes Class 9 Science Chapter 8 Force and Laws of Motion

Notes For All Chapters Science Class 9

1. Introduction

Force:

• Force is a push, pull, or hit that can:
• Change the state of motion of an object (speed up or slow down)
• Change the direction of motion
• Change the shape or size of an object

Effects of Force:

• Pushing a trolley moves it forward.
• Pulling a drawer opens it.
• Hitting a ball changes its motion.

2. Balanced and Unbalanced Forces

Balanced Forces

• When equal forces act on an object in opposite directions.
• These do not change the state of rest or uniform motion.

Example:

• If a block is pulled equally from both sides, it remains stationary.

Unbalanced Forces

• When two opposite forces are unequal.
• Causes a change in motion (object accelerates).

Example:

• Children pushing a box on the floor — it moves only when the pushing force becomes greater than friction.

Role of Friction

• Friction is a force that opposes motion between two surfaces in contact.
• When we stop pedalling a bicycle, it slows down due to friction and air resistance.
• A moving object continues to move uniformly if no unbalanced force acts on it.
• To accelerate an object, an unbalanced force is required.

3. First Law of Motion (Law of Inertia)

Galileo’s Observation

• Galileo studied the motion of balls on inclined planes and found that:
  • On a downward slope, velocity increases.
  • On an upward slope, velocity decreases.
  • On a frictionless horizontal surface, a ball would continue moving forever (no force needed to maintain motion).

Conclusion:

• A force is needed only to change motion, not to maintain it.
• In real life, objects stop because of friction and air resistance.

Newton’s First Law of Motion

“An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an unbalanced external force.”

Meaning:

• Objects resist changes to their motion.
• This property of resistance is called Inertia.
• Hence, the First Law is also called the Law of Inertia.

Examples (Inertia in Daily Life):

• When a car suddenly stops, passengers move forward → inertia of motion.
• When a bus starts suddenly, passengers fall backward → inertia of rest.
• During a sharp turn, body tends to continue in a straight line → inertia of direction.
• Seat belts apply force opposite to motion to reduce injury.

4. Inertia and Mass

Inertia – Definition

“The tendency of an object to resist a change in its state of rest or motion.”

• Every object resists changes to its state.
• The greater the mass, the greater the inertia.

Relation between Mass and Inertia

Mass is a measure of inertia.
→ Heavier bodies are harder to move or stop.

Examples:

• Easier to push an empty cart than a loaded one.
• A stone has more inertia than a rubber ball of the same size.
• A train has much more inertia than a bicycle.

Types of Inertia

1. Inertia of Rest – Body resists starting motion.
2. Inertia of Motion – Body resists stopping or changing speed.
3. Inertia of Direction – Body resists change in direction.

5. Second Law of Motion

When an unbalanced force acts on a body, it produces acceleration.
The acceleration depends on:

• The force applied, and
• The mass of the object.

Momentum

“The product of the mass and velocity of an object.”

p = m×v

Vector quantity (direction same as velocity)

SI Unit: kg·m/s

Example:

• A heavy truck moving slowly and a small stone moving fast can have the same momentum.

Newton’s Second Law

“The rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of the force.”

Mathematical Form:

Let,
Mass = m, Initial velocity = u, Final velocity = v, Time = t

Unit of Force:
1 newton (N) = Force that produces an acceleration of 1 m/s² in a 1 kg mass.

Examples of Second Law:

1. A cricketer pulls hands backward while catching a fast ball → increases time → reduces force.
2. Cushioned beds in high jump → increase time of impact → reduce injury.
3. Car airbags work on the same principle.

Numerical Example:

Example 8.1:

m = 5 kg, u = 3 m/s, v = 7 m/s, t = 2 s

Key Points:

• Force = rate of change of momentum.
• Greater change of momentum in less time → greater force.
• If F = 0, velocity remains constant → First Law derived from Second Law.

6. Third Law of Motion

Newton’s Third Law

“To every action, there is an equal and opposite reaction.”

• Forces occur in pairs.
• Action and reaction are equal in magnitude, opposite in direction, and act on different bodies.

Examples:

• Walking: Foot pushes ground backward → ground pushes forward.
• Rowing boat: Oars push water backward → boat moves forward.
• Gun and bullet: Gun recoils backward when bullet is fired.
• Jumping from a boat: Boat moves backward when sailor jumps forward.
• Football collision: Both players experience equal and opposite forces.

Important Formula: