Forces and Motion

Newton's Three Laws of Motion

First Law — The Law of Inertia

An object at rest stays at rest, and an object in motion stays moving at a constant velocity, unless acted on by a resultant force. In simple terms: things keep doing what they are already doing until something pushes or pulls them. A book sitting on a table will not move unless you push it. A ball rolling on a perfectly smooth surface would roll forever if nothing slowed it down.

Second Law — F = ma

The resultant force on an object equals its mass multiplied by its acceleration. A bigger force gives a bigger acceleration. A heavier object needs more force to achieve the same acceleration. The equation is F = m x a, where F is force in newtons, m is mass in kilograms and a is acceleration in metres per second squared (m/s²).

Third Law — Action and Reaction

For every action there is an equal and opposite reaction. When you push against a wall, the wall pushes back on you with exactly the same force. When a rocket expels gas downwards, the gas pushes the rocket upwards. These pairs act on different objects, which is why they do not cancel out.

Speed, Distance and Acceleration

Speed tells you how fast something is moving. The equation is speed = distance / time. If a cyclist travels 200 metres in 40 seconds, their speed is 200 / 40 = 5 m/s. Acceleration tells you how quickly speed is changing. The equation is acceleration = change in velocity / time. If a car speeds up from 0 to 20 m/s in 10 seconds, its acceleration is (20 - 0) / 10 = 2 m/s².

Friction and Terminal Velocity

Friction is a contact force that opposes motion. It acts between any two surfaces that slide over each other and converts kinetic energy into thermal energy. Air resistance is a type of friction that acts on objects moving through the air.

Imagine a skydiver jumping from a plane. At first, the only force acting is weight (gravity pulling down), so they accelerate. As their speed increases, air resistance increases too. Eventually air resistance equals weight — the forces are balanced, the resultant force is zero and acceleration stops. The skydiver now falls at a constant speed called terminal velocity. When the parachute opens, air resistance suddenly increases far beyond the weight, creating an upward resultant force that decelerates the skydiver until a new, much lower terminal velocity is reached.