First law — inertia
An object at rest stays at rest, and an object in motion keeps moving at the same speed in the same straight line, unless an unbalanced force acts on it.
This is why you lurch forward when a bus brakes: the bus stops, you keep moving. It is also why a puck slides a long way on ice — with little friction, there is almost no unbalanced force to stop it.
Second law — F = ma
The acceleration of an object is proportional to the net force on it and inversely proportional to its mass: F = m × a.
Example: a net force of 20 N acts on a 4 kg box. a = F/m = 20/4 = 5 m/s². Example: to accelerate a 1200 kg car at 2 m/s² you need F = 1200 × 2 = 2400 N.
The word 'net' matters. If a 30 N push meets 12 N of friction, use 18 N in the formula.
Third law — action and reaction
For every action there is an equal and opposite reaction. The two forces are always the same size, always opposite in direction, and always act on two different objects.
A swimmer pushes water backwards; the water pushes the swimmer forwards. A rocket pushes gas down; the gas pushes the rocket up. Because the forces act on different objects, they never cancel each other out — that is the point most students miss.
Which law is the question about?
Use these cues to pick the right law before you start calculating.
- Nothing is accelerating, or something suddenly stops or starts → first law.
- Numbers for force, mass or acceleration appear → second law, F = ma.
- Two objects pushing on each other → third law.