To protect drivers and passengers from serious injury in the event of a crash or other collision, cars have safety features built in:

• air bags
• crumple zone
• inertia wheel safety belts

All of these prolong the time of impact, reducing the force on the occupants.

Using Newton’s Second Law the force on the car in a collision will be given by this formula.

The safety features won’t change the mass of the car or the starting and end speeds, but do increase the impact time t in the formula. As impact time increases, the force on the occupants gets less as you are dividing by a higher value.

Speed limits in areas where there are lots of pedestrians – such as schools – are lowered to 20 miles per hour, as the force and kinetic energy of a vehicle in an impact are much reduced. When the speed of a car doubles, the impact force will be doubled and the kinetic energy will be quadrupled, leading to the possibility of fatal collisions with pedestrians and drivers.