In an emergency, a driver must bring their vehicle to a stop in the shortest distance possible:
stopping distance = thinking distance + braking distance
This is when:
Reaction times vary from person to person, but are typically 0.2 s to 0.9 s. A driver's reaction time can be affected by:
Longer reaction times increase the thinking distance when stopping from a given speed.
There are different ways to measure reaction times. One simple method involves dropping a ruler between someone's open thumb and forefinger. The higher the reaction time needed to grasp the falling ruler, the further the ruler falls before being stopped.
The braking distance of a vehicle can be affected by:
The faster a vehicle travels, the greater the braking force needed to stop it in a certain distance. A greater braking force produces a greater deceleration. Large decelerations may cause the brakes to overheat, and the driver may also lose control of the vehicle.
It is important to be able to:
The diagram shows some typical stopping distances for an average car in normal conditions.
It is important to note that the thinking distance is proportional to the starting speed. This is because the reaction time is taken as a constant, and speed = distance × time. However, the braking distance increases by a factor of four each time the starting speed doubles.
Car safety features such as seatbelts, airbags and crumple zones all work to change the shape of the car, which increases the time taken for the collision. Crumple zones refer to the areas of a car that are designed to deform or crumple on impact. These different safety features decrease the force of the collision on any people within the car.
During a collision there is a change in momentum. The force of the collision is equal to the rate of change of momentum. The safety features decrease the rate of change of momentum, which decreases the force of the collision on any people within the car.
When cars are involved in accidents, the forces exerted are very large. Crumple zones are designed to increase the time it takes for a car to stop totally. This reduces the force acting on a car. Seat belts and air bags are designed to increase the time it takes for passengers in a car to stop totally. This reduces the force acting on the passengers.
Consider two cars colliding.
The mass of each car is 1,000 kg and they are both travelling at 20 m/s (72 km/h).
Because momentum is conserved, they are both stationary after the impact.
average velocity = (20 + 0) ÷ 2 = 10 m/s
During a collision, the crumple zone is shortened by 0.50 m.
Using the equation: