Safety on our roads has been an important issue in our society. Engineers have been altering car designs to give us maximum safety in car accidents. Airbags, seatbelts and structure of the car are safety factors after collisions. But what happens before collisions? From the statement; safety is not related to how fast you go but how fast you stop. To find the truth, we must investigate…
To understand this statement, we must grasp the concepts of physics. Momentum is defined as mass in motion. It is the product of the mass and the speed it is travelling, so if we want to increase the momentum, we increase mass or the speed. To change the momentum of an object, we must provide an impulse. Impulse is the product of force and time, and equals change in momentum.
If we have 2 cars: A and B with the same mass, but A travelling half the speed of car B. Using our common sense, we can predict car B to suffer more damage during a head-on collision with the wall. Why? Using our knowledge of physics, car B has more momentum, because speed is increased. In the same situation, both cars will undergo changing momentum to zero. Since car B has more momentum, its change in momentum is larger. As a result, more opposing impulse is needed to decrease it. More impulse means more of the product of force and time. Braking is an option, which provides more impact time, but may not be enough to increase safety.
During collision, the wall provides an impulse to stop the car. Since we are colliding with something solid, not soft like a haystack, (which also have the same impulse with the product of a larger time of impact and less force), but in this case the time of impact is minimised and forces larger. Due to Newtons Third Law, this force is felt by the car as well as the wall. In conclusive, the slower you go, the less impulse acted on your car, thus safer.
The safety of the car and the driver is also determined by how fast you stop. During a head-on collision with the wall, the only option is to brake immediately to decrease the momentum. If brakes are unable to stop the car as it continues moving towards the wall, then the impulse would be lesser than without braking at all. In terms of physics, the earlier you brake, the earlier you provide the impulse to change the momentum to zero. The impulse, which is the car braking, can increase the time of impact during the collision with the wall. As a result, the force of impact is reduced, because impulse is always the same.
This can be demonstrated in a collision between a wall and a haystack. During the collision of the haystack, a lesser force is experienced on the car because the time of impact increased. Oppositely, hitting a wall has the same impulse or change in momentum as the haystack collision, however, the time is reduced and the force is enormous. In both situations their impulses of force times time are the same. So braking earlier is advantaged by increasing impact time.
Looking at both sides of the argument, safety is related to both how fast you go and stop. In todays generation of car accidents, speeding has resulted in many tragic deaths. In driving laws, we have noticed why we have speed limits in our roads. Obviously, in case of accidents. I believe it is more safe to slow down, due of many factors. Therefore, I disagree with this statement.
Trying to brake as fast as possible is a big gamble. For the safety of your life and also others, betting on your reaction rates of stopping are very risky. Even the most sensitive person may find it hard to avoid a collision, as sliding distance is incontrollable. Other factors such as a heavier car will require more braking distance, as more change in momentum is needed.
Since you are travelling slower, before the situation of crashing, you are likely to have more thinking time for you options. A higher speed will decrease the time needed to react and hit the brakes, whilst a slower speed can supply you more time to react and brake longer. Lowering the speed means momentum is less; thus the change in momentum to zero can be achieved more quickly. Even if a collision does take place, the force of impact will not be as great as a high-speed collision.
An after collision for an unsuccessful stop is harmful. The use of airbags and seatbelts inside cars can save lives, but sometimes we cannot rely on them too much. If we consider a side-collision, the driver and the passengers are more at risk.
In summary, safety is related to many factors. How fast you stop, how fast you go, what car is used and the special features inside the car; all these things really depend on the situation. If we say it is safe to go fast, the reason might be because you are colliding with a haystack. We have learnt that physics plays a big role in car safety and prevention of collisions. There are no real answers to what makes driving safe, so for now we must rely on our natural instincts of physics…