Motion and forces Study Guide

Large decelerations are extremely dangerous because they involve a rapid decrease in velocity over a short time, resulting in very large forces. According to Newton’s second law, the force acting on an object equals the rate of change of momentum: F = Δp/Δt, where Δp is change in momentum and Δt is the time taken for this change. In road accidents, if the time over which a vehicle stops is very short, the force on occupants becomes extremely high, leading to serious injuries such as fractures, organ damage, and concussions. Safety features such as seatbelts, airbags, and crumple zones are designed to increase the time over which momentum changes, thereby reducing the force and making collisions survivable.

The distance a road vehicle needs to stop in an emergency is influenced by several factors, most notably its initial speed. Stopping distance is comprised of two parts: thinking distance and braking distance. Thinking distance is the distance travelled while the driver reacts to a hazard and applies the brakes. Braking distance is the distance travelled from the moment the brakes are applied until the vehicle comes to a complete stop. As speed increases, both thinking distance and braking distance increase, but not linearly. Thinking distance is directly proportional to speed, as the reaction time remains relatively constant. However, braking distance is approximately proportional to the square of the speed. This quadratic relationship means that doubling the speed quadruples the braking distance, making high speeds significantly more dangerous in emergency stopping situations.