Newton's Laws and car safety

Car safety features

Newton's Laws are very important when it comes to car safety.

When there is a car crash, the car, its contents and the passengers decelerate rapidly. They experience great forces because of the very large decelaration, which can cause injury.

Modern cars also have safety features that absorb kinetic energy in collisions. These typically include:

  • seat belts
  • air bags
  • crumple zones

These features increase the time taken for the change in speed of the occupants. This reduces the deceleration, which causes the forces involved to be reduced, and consequently serious injuries to be reduced.

Seat belts

Seat belts stop you tumbling around inside the car if there is a collision. Upon sensing a collision the seat belts lock in place. When the car crashes, there is no unbalanced force acting on the person, so they continue forward (Newton's First Law). The person moves against the seat belt, exerting a force on it. The seat belt then exerts a force back on the person (Newton's Third Law). This causes a controlled deceleration of the person.

Air bags

Air bags increase the time taken for the motion of a car occupant's head to decelerate from maximum speed to zero. A short sharp deceleration would involve a very large force, increasing the chance of a head injury. A longer time slowing down decreases deceleration thereby reducing the size of the force acting and decreasing the chance of injury.

Crumple zones

Crumple zones are areas of a vehicle that are designed to crush in a controlled way in a collision. They increase the time taken for the vehicle to slow down in an impact (like an airbag). This reduces the force exerted on the passengers. The deformation (crumpling) of the car also absorbs energy from the collision meaning that less energy is transferred to the passengers.

A car which has crashed, showing the front and rear crumple zones, seat belts and activated airbags.

Driving safely

The above safety features are there to reduce the chance of injury when an accident occurs. Ideally responsible drivers should do the following to decrease the chance of an accident.

  • Not exceed the speed limit (greater speed = greater kinetic energy = greater chance of injury in an accident).
  • Pay careful attention to other road users.
  • Obey the Highway Code.
  • Refrain from using mobile phones/ipods while driving which take driver's concentration away from driving carefully.
  • Drive to the conditions of the road – the speed limit might be 30 mph but in icy conditions the stopping distance will be increased.
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