High School Physics

Car safety, momentum & force – GCSE Physics

Last updated on October 2nd, 2021 at 09:48 am

Car safety, momentum & force (GCSE Physics) – Here, we will discuss how momentum and force in physics can influence car safety and its design.

The force on a person in a car crash can be very large and the force can be worked out from the deceleration in a crash.

You can also work out the force in a crash using the equation for momentum.

Car safety is designed using the concept of momentum and force related to a car crash. The objective is to keep the force on a person as small as possible.

Let’s take an example of a car travelling a steep road.

car safety on steep roads using physics & escape lanes

Steep roads often have escape lanes filled with deep, soft sand. The soft sand slows heavy cars that are out of control slowly – by making the time duration for the vehicle to stop longer. We know that the force, F, is the rate of change of momentum i.e the ratio of change in momentum and the time duration. As the time duration for the car to stop is made longer the force that slows the car gets smaller and the driver is less likely to suffer serious injury,

sample numerical on car crash and force experienced by the passenger

Let’s take one numerical problem to understand how to calculate this force.

Question: A car travelling at 20 m/s collides with a stationary lorry and is brought to rest in just 0.02 s. A woman in the car has a mass of 50 kg. She experiences the same deceleration when she comes into contact with a hard surface in the car (such as the dashboard or the windscreen). What force does the person experience?


Force = rate of change in momentum = change in momentum/time duration
=[ (50 kg x 20 m/s) – (50 kg x 0 m/s) ] / 0.02 s
= 50000 N

safety features of car, crumple zone and time for momentum change

Cars are now designed with various safety features that increase the time over which the car’s momentum changes in an accident.

The car has a rigid passenger cell or compartment with crumple zones in the front and behind. The crumple zones, as the name suggests, collapse during a collision and increase the time during which the car is decelerating.

For instance, if the deceleration time in the above example is increased from 0.02 s to 1 s, then the impact causes a much smaller force of just 1000 N to act on the passenger, greatly increasing their chances of survival.

Crumple zones are just one of the safety features now used in modern cars to protect the passengers in an accident.

Crumple zones only work if the passengers are wearing seat belts so that the reduced deceleration applies to their bodies too. Without seat belts, the passengers will continue moving forward until they come into contact with some part of the car or with a passenger in front.

If they hit something that does not crumple they will be brought to rest in a very short time, which means a large deceleration and, therefore, a large force acting on them.

Know more in another related post about different applications of impulse in physics to improve safety.

See also  Coulomb Force Strength versus Gravitational Force strength
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