Forces and Newton's laws

Force

A push or pull, measured in newtons (N). Force is a vector (has size and direction).

Weight = mass x gravitational field strength (W = mg), with g ≈ 10 N/kg on earth. A 50 kg student weighs 500 N.

Newton's first law (inertia)

An object stays at rest or moves at constant velocity unless acted on by a resultant force. This is why seat belts are important: passengers keep moving forward when a car stops suddenly.