The physics of auto accidents

8 highway deaths mar Memorial weekend

One type of energy that physicists like to keep track of is kinetic energy, which is the energy associated with movement. A body in motion has more energy than the same body at rest.

Kinetic energy is generally proportional to the square of the velocity (from the equation E=(1/2)mv^2). So you have twice as much energy when you are going 70 mph as when are you going 50 mph.

Force, in physics, is generally thought of as the ability to change the velocity of something over time. So it takes more force to slow something down quickly and less force to slow something down slowly.

If you get in a car accident, what you hope is that your kinetic energy will be a) dissipated by something other than you and b) dissipated over as much time as possible.

Letter ‘a’ above is why we have airbags and crash zones in cars. They absorb energy. It’s also why we have seat belts. If you get ejected from the car, you may slow down a bit slower (b), but you absorb all the energy yourself. This is the functional equivalent of jumping off of a building.

Letter ‘b’ is why it is often a good thing if you roll a car, because the energy dissipates more slowly. You have to be in a seat belt to take advantage of this, though. Rolling a car converts some of the energy to rotational kinetic energy. People are routinely hurled unceremoniously through windshields and side windows of cars only to be slammed into the ground at 50 mph.

So rather obviously, head-on collisions are by far the worst. The energy gets dissipated much too quickly and a ton of force is applied to you, the occupant. So stay sober and stay between the damn lines because crossing the center line kills people dead.

But most importantly, wear your seat belt. Your odds of surviving go way, way up if you stay in the car. When you leave the car you almost always die, due to both ‘a’ and ‘b’ above.

If you are wearing your seat belt and you have to make a quick decision whether to run into a wall or roll the car, roll the car.

The physics of auto accidents

One thought on “The physics of auto accidents

  1. bsherwood says:

    I pick “roll the car” every time.

    You should do the same NRG study to determine why a motorcycle rider without a helmet automatically lands on his head. The rider “with” a helmet often times does not….or maybe it is because the helmet head will bounce where the exposed melon does not??


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