Car Safety

There are a number of safety devices that can be used to reduce the chances of a serious injury in a car when a collision occurs.  These can also be related quite easily to Newtons Laws.

1. Air bags (Newtons 2nd Law describing momentum).

While many of us thing that Newtons 2nd Law is just about F=ma, Newton in his original description stated 

An applied force is equal to the rate of change of momentum.

Recall that impulse is the amount of force applied over a given time.  

This will change the momentum of the object.  When a collision occurs, a change in momentum is inevitable. But it is dependent on the time taken.  So if we increase the time that this change in momentum takes, we can reduce the force that can affects people inside the car.  This is where airbags go to work.



Apart from reducing the applied force, airbags spread the force across a larger area meaning that rather than being concentrated at one point of impact, it will be more diffuse or spread out.  So while damage or injury may still occur, it will be less, but spread out more.


Crumple zones work on a similar principle wit regards to impulse and momentum.  They also have the added benefit if absorbing a lot of the energy in a collision and as a result, rather than an elastic collision where cars would bounce off each other, resulting whiplash injuries, they reduce this by having steering systems fold up and in this case the front end of the car distorting.




Seat belts have a lot more to do with Newton's 1st Law dealing with inertia.  While a person is in the car they are not part of the car.  Thus when the car stops, the person will want to keep moving.  In the case of sudden stops the effect becomes more pronounced.  Seat belts made of wide webbing again will apply a force on you to stop you from flying out of the seat in a collision, but they do it over a larger area and so reduce the chances of serious injury.  The spreading of force can be enhanced by widening the seat belt even further as pictured on the right here.




Unsecured objects can be seen to have very dangerous effects as shown by the picture below.  Inertia becomes more problematic as the mass increases. Even relatively small items such as torches, pieces of sporting equipment or shopping are potentially lethal in a collision because the will continue moving.  One blow to the head could be debilitating or even fatal.