Newton's Laws and Forces

Newton's First Law (inertia)

An object stays still, or moves at constant velocity, unless acted on by a resultant force. Balanced forces → velocity unchanged.

Newton's Second Law

force = mass × acceleration      F = m × a

Bigger force → bigger acceleration; bigger mass → smaller acceleration.

Newton's Third Law

Every action has an equal and opposite reaction; the pair acts on different objects.

Weight

weight = mass × gravitational field strength      W = m × g    (g ≈ 9.8 N/kg)

Hooke's law (springs)

force = spring constant × extension      F = k × e

Valid up to the limit of proportionality; beyond it the spring deforms permanently.

Work done & momentum

work done = force × distance      W = F × s   (joules)
momentum  = mass × velocity       p = m × v   (kg m/s)

Momentum is conserved in collisions.

Terminal velocity

A falling object accelerates until air resistance balances weight (resultant force = 0), then falls at a steady terminal velocity.

Exam tip

"Resultant force" decides acceleration. Balanced forces = constant velocity (including stationary). Use F = ma whenever you're given two of force, mass, acceleration.