Newton's laws, weight and stopping distances

Newton's second law: resultant force = mass × acceleration, F = m a. Weight is the force of gravity on a mass: W = m g, with g about 9.8 N/kg on Earth. Inertia is the tendency of an object to stay at rest or keep moving at constant velocity; inertial mass is a measure of how hard it is to change its velocity.

A vehicle's stopping distance is the thinking distance plus the braking distance. Thinking distance increases with speed and with slower reactions caused by tiredness, alcohol, drugs or distractions such as phones. Braking distance increases with speed, wet or icy roads and worn tyres or brakes. Doubling speed roughly quadruples the braking distance, because the kinetic energy that the brakes must remove depends on v².