what is the function of seat belts and air bags in automobiles? need a physics answer!?

Simply put, to save your life by increasing the time in which you come to a complete stop.

We can first look at the seatbelt scenario. Seat belts come in two varieties, the stretching and non-stretching kinds. Since you want a Physics answer here goes.

Let’s look at a car that’s moving at about 30 miles/hr or about 13.4 m/s. Let’s also assume that we have a man in the car whose mass is about 70 kg.

If there is no seatbelt, the distance the driver moves in a crash is determined by the distance from the steering column to the windscreen. This means the driver crashes, moves forward to the steering column and then starts decelerating. He hits the windscreen. This distance is typically about a few inches, lets say about 0.2 ft.

So

v^2 = u^2 +2(a)(s)

We get a deceleration of about

a= -1500ms^-2

This is about 150g’s

Now what a seatbelt does is stop you with the car. This will typically give you a stopping distance that is about 4 to 5 times more than if you weren’t wearing the seatbelt at all!

Let’s say that the stopping distance is about 0.3m

Using the above equation, we find that we get a deceleration of about 300ms^-2. This is about 30g’s

Now for the stretching seatbelt case. the stretching seatbelt extends the distance under which the driver stops. You cna see from the above this is a good thing. The seatbelt can typically increase the stopping distance by about 50%. So the stopping distance from above increases to 0.45m

Again plugging these values into the above equation we get a deceleration of about 200ms^-2. This comes to about 20g’s

So the function of the seatbelt is to increase the stopping distance of the driver or passenger. Thisreduces the deceleration of the person and hence the forces they experience.

So what’s the deal with the airbag??

Well, the airbag doesn’t decrease the forces on the passenger or driver. The forces will be the same. In a crash the seatbelt is strapped across the person’s ribcage. This is where most of the forces are centered. The airbag is basically a fluid filled bag and so it allows the force to be distributed over a greater area. This reduces the pressure on the body.

Now this does not mean that you should not use a seatbelt. The seatbelt also prevents you from flying about as the car crashes. The airbag will help in cushioning the impact.

Hope this helps.

Automobile air bag deployment

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