9th Class Physics Chapter 3 Notes PDF 2026 – Dynamics

Rule

First law (law of inertia): a body stays at rest, or moves with constant velocity in a straight line, unless acted on by a net external force.

Second law: $F = ma$ — the net force equals mass times acceleration.

Third law: for every action there is an equal and opposite reaction, acting on two different bodies.

Definition

Inertia is a body's tendency to resist a change in its state of rest or motion — more mass means more inertia.

One newton is the force that gives a $1$ kg mass an acceleration of $1$ m/s $^{2}$ .

Example 1 — Newton's second law

A force of $20$ N acts on a body of mass $4$ kg. Find the acceleration.

$F = ma \Rightarrow a = \frac{F}{m} = \frac{20}{4}$ .

Answer: $5$ m/s $^{2}$

Remember

A passenger lurches forward when a bus stops suddenly because their body's inertia keeps it moving forward even though the bus has stopped — this is why seat belts matter.

Momentum $p = m v$ (SI unit: kg·m/s)

Law of conservation of momentum: in a system with no net external force, total momentum before an interaction equals total momentum after it.

Example 2 — calculating momentum

Find the momentum of a car of mass $1000$ kg moving at $20$ m/s.

$p = m v = 1000 \times 20$ .

Answer: $20, 000$ kg·m/s

Example 3 — recoil

A $20$ g bullet is fired from a $4$ kg gun at $300$ m/s. Find the gun's recoil velocity.

Momentum before firing $= 0$ (both at rest).
By conservation: $m_{bullet} v_{bullet} + m_{gun} v_{gun} = 0$ .
$(0.02) (300) + 4 v_{gun} = 0 \Rightarrow v_{gun} = \frac{- 6}{4}$ .

Answer: $- 1.5$ m/s (recoils backward)

Definition

Friction is the force that opposes relative motion between two surfaces in contact, and always acts opposite to the direction of motion.

Coefficient of friction $μ = \frac{F}{N}$ ( $F$ = friction force, $N$ = normal reaction).

Remember

Friction is called a "necessary evil": it wastes energy as heat and wears out machine parts, but without it we couldn't walk, and vehicle brakes and tyres wouldn't grip. Lubricants and ball bearings reduce friction between moving machine parts.

Example 4 — friction force

A $20$ kg block rests on a surface with $μ = 0.3$ . Find the friction force. ( $g = 10$ m/s $^{2}$ )

$N = m g = 20 \times 10 = 200$ N.
$F = μ N = 0.3 \times 200$ .

Answer: $60$ N

Quick Recap

●Newton I: inertia keeps a body's state unchanged without a net force. Newton II: $F = ma$ . Newton III: equal, opposite, on two different bodies
●Momentum $p = m v$ ; total momentum is conserved when no net external force acts
●Friction opposes relative motion; $μ = \frac{F}{N}$
●Friction is a "necessary evil" — reduced by lubricants and ball bearings

9th Class Physics Chapter 3 Notes

Newton's Laws of Motion

Momentum and Its Conservation

Friction

More 9th Class Physics notes

Physical Quantities and Measurements

Measurements

Kinematics

Kinematics

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Turning Effects of Force