Today, we constantly see in the media that the number of traffic accidents has increased, whether they are caused by human errors, lack of road maintenance or mechanical failure of the automobiles. What we see is that more and more the auto industry is being forced, both by government regulatory measures and by issues of marketing, to adopt safety mechanisms that can better protect vehicle passengers in the event of a collision.
If we look closely, the most common measures that industries are taking are in relation to building cars with safer structures, such as airbags, stronger and more efficient seat belts and safer seats.
When a collision occurs, whether between two vehicles or between a vehicle and a fixed structure (wall, for example), there is always a variation in the amount of movement of the car's occupants. For example, let's assume that the total mass (car + passengers) of a vehicle is 800 kg and that it is traveling at a speed of 15 m/s (54 km/h). The amount of movement of this set is 12,000 kg.m/s. A driver who has a mass of 70 kg, inside the car, will have a movement amount of 1,050 kg.m/s.
Now let's suppose the car collides with a wall. In a very short time, the speed goes to zero and, thus, the impulse that the driver suffers should be 1,050 kg.m/s, which is nothing more than his variation in the amount of movement. The greater the speed of the vehicle before the collision, the greater the thrust needed to stop the driver.
The way the vehicle is built is a determining factor for the safety of its occupants. The industry seeks to develop projects that allow the longest possible collision time, since for each collision the product 
.t will be constant. The longer the collision time, the less force will be and therefore the lesser the chance of damage to the car's occupant. In addition to improving the car's structure, safety devices can also be installed.
One of the most efficient equipment to prevent injuries from crashes is the air bag. Placed between the front seats and the dashboard or on the sides, it quickly inflates when a violent deceleration occurs. In the case of head-on collisions, the driver crashes into the air bag, which is much more flexible than the dashboard.
Consider two identical collisions, but bear in mind that in only one of the situations the car has an airbag. The driver x airbag collision lasts much longer than the driver x panel collision. For both cases, the variation in the driver's amount of movement is the same, but the time it takes to stop is much longer in the situation with air bag, thus resulting in less strength. In numerical terms, the air bag can increase collision time up to ten times. Typical stop times would be 0.05 seconds without airbag and 0.5 second with air bag. With these times and the data above, we would have for the driver:
Pbefore=mvbefore
Pbefore=70 x 15=1,050 kg.m/s
Plater=0
∆p=0-1.050=-1,050 kg.m/s
Like
With airbag, we would have:
and without air bag:
The force acting on the driver is ten times smaller when comparing the situation with and without air bag. The force calculated above is the average force acting during the time interval that the collision with air bag or with the dashboard.
By Domitiano Marques
Graduated in Physics
