Imagine a car travelling along a road when it suddenly crashes. The car may stop in a fraction of a second, but the passengers inside were moving at the same speed and therefore have momentum. Seat belts, airbags and crumple zones are designed to control how that momentum changes during a collision. Momentum depends on an object's mass and velocity. To bring a moving passenger to rest, their momentum must change. A force acting over a period of time produces an impulse, and impulse is equal to the change in momentum. This gives engineers an important safety principle: for the same change in momentum, increasing the time over which the change happens reduces the average force. An airbag inflates rapidly during a serious collision. Instead of the passenger stopping against a hard dashboard or steering wheel, the airbag allows them to slow down over a greater distance and a longer time. This reduces the average force acting on them. The airbag also spreads contact forces over a larger area of the body. Airbags are designed to work alongside seat belts, not replace them. Crumple zones use a related idea. Parts of a car are deliberately designed to deform during a collision. This deformation transfers kinetic energy into other forms and increases the time over which parts of the vehicle change momentum. Meanwhile, the passenger compartment is designed to remain much more rigid to protect the people inside. The same physics appears in many other situations. A climbing rope stretches when a climber falls, a crash helmet contains materials that deform during an impact, and a goalkeeper moves their hands backwards while catching a fast-moving ball. Each action can increase the stopping time and reduce the average force. Engineers cannot make momentum disappear during a collision. Instead, they use physics to control how quickly momentum changes and where forces act. A fraction of a second can make an enormous difference.