Newton's First Law — The Law of Inertia

> An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force.

Inertia is the tendency of an object to resist changes in its state of motion. The greater an object's mass, the greater its inertia. This is why it takes more effort to push a fully loaded shopping trolley than an empty one — the loaded trolley has more mass and therefore more inertia.

Real-world example: When a car brakes suddenly, passengers lurch forward. Their bodies tend to continue moving at the car's original speed because of inertia.

Newton's Second Law — Force and Acceleration

> The net force acting on an object equals the object's mass multiplied by its acceleration.

$$F = ma$$

where $F$ is net force (N), $m$ is mass (kg), and $a$ is acceleration ($\text{m/s}^2$).

Worked example 1: A person pushes a 10 kg box, producing an acceleration of $2\ \text{m/s}^2$. The required net force is:<br>$$F = ma = 10\ \text{kg} \times 2\ \text{m/s}^2 = 20\ \text{N}$$

Worked example 2: A 5 kg object experiences a net force of 15 N. Its acceleration is:<br>$$a = \frac{F}{m} = \frac{15\ \text{N}}{5\ \text{kg}} = 3\ \text{m/s}^2$$

Newton's Third Law — Action and Reaction

> For every action force, there is an equal and opposite reaction force.

Action-reaction pairs always act on different objects. They are equal in magnitude and opposite in direction.

Examples of action-reaction pairs:

• A rocket engine expels gas downward (action); the gas pushes the rocket upward with equal force (reaction). This is thrust.
• When you push against a wall (action), the wall pushes back on your hand with equal force (reaction).
• A swimmer pushes water backward with their hands (action); the water pushes the swimmer forward (reaction).

Important: Action and reaction forces do not cancel each other out — they act on different bodies.