Seatbelts
Uploaded by Buttwhisker on May 25, 2000
When travelling at slow speeds in your car the wearing of a seatbelt has little effect of your body when you brake. So why is it important to wear your seat belt?
A driver or passenger travelling in a car is moving at the same speed as the car. If the car suddenly stops, the body of the rider inside will keep moving forward at the same speed. This demonstrates inertia. The tendency of a moving object to keep moving, or of a stationary object to remain at rest. Basically Newtons first law; that a body stationary or moving with constant velocity will want to continue to do so, unless acted on by a force.
Lets understand what is happening here. First drive along in your car at 60 km/h on a backstreet with no traffic, then brake gently and slowly. You will notice that the seat belt doesn’t really do much to hold your body. Now do the same again but this time break as quickly and sharply as you can. Your body will be thrown forwards with great force, and your seatbelt will be literally holding you in place.
Now your body was what is commonly referred to as being "thrown forwards", however this is not the case. Your body was actually not slowing down much at all and your velocity relative to the car initially was much greater. The car began to slow down due to breaking and your body in accordance with Newtons First law wanted to continue to move at the original constant velocity. Now if your seat belt was not there to provide an opposing force, to your momentum and inertia, by holding you from going forwards, you very likely would have been thrown into the dash or steering wheel.
Lets look at this mathematically.
m= your mass in kilograms for this purposes 70kg
V= final velocity 0 m/s
U= initial velocity 60 km/h or 16.6 m/s straight line
S= distance taken to stop 42 m
t= 3.8
a= -4.368 m/s/s
Now your momentum at 60km/h is P=MU
So P= 70kg*16.6m/s
P=1162 Kg m/s
Impulse I=MU/t
I=70*16.6/3.8
I=305N
So your body will weigh about 610kg when you are breaking hard, a force it is difficult for any person to withstand.
Now in the context of a head on accident at around 60km/hr the force exerted on your body is greatly increased. In the event of such an accident it will take the car approximately 0.4 seconds to stop. This...