Newton's three laws of motion describe how objects move and respond to forces. Published in 1687, they still explain almost everything you see moving around you: from a stationary book to a football in flight.
First Law: Inertia
An object at rest stays at rest. An object in motion continues in a straight line at constant speed. This continues UNLESS an unbalanced external force acts on it. Inertia: the tendency of an object to resist changes in its state of motion. Heavier objects have greater inertia. Example: When a bus brakes suddenly, passengers lurch forward: they were in motion and their inertia kept them moving while the bus stopped.
Second Law: F = ma
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Formula: F = ma where F = force (Newtons, N), m = mass (kg), a = acceleration (m/s²) Example: A 5 kg object accelerates at 3 m/s². What force is applied? F = 5 × 3 = 15 N Example: A force of 20 N acts on an 8 kg object. What is the acceleration? a = F/m = 20/8 = 2.5 m/s²
Third Law: Action and Reaction
For every action, there is an equal and opposite reaction. The forces act on DIFFERENT objects: that is why they don't cancel out. Example: A rocket fires exhaust gases downward (action). The gases push the rocket upward (reaction). Example: You push on a wall (action). The wall pushes back on your hand (reaction).
Key points
- 1st Law: no force = no change in motion (inertia)
- 2nd Law: F = ma (force, mass, acceleration)
- 3rd Law: equal and opposite reaction on a DIFFERENT object
- Weight = mg (mass × gravitational field strength)
- Net force = the vector sum of all forces acting
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WAEC tip
WAEC often combines Newton's 2nd Law with weight (W = mg) in a single calculation. Keep mass (kg) and weight (N) distinct: they are different quantities.
Practise this
A 1,200 kg car accelerates from rest to 20 m/s in 8 seconds. (a) Calculate the acceleration. (b) Calculate the force needed. Assume no friction.
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