QUESTION: Which is worse; my car running into a wall at 50 mph, or my car going at 50 mph hitting a car coming at me at 50 mph?

ANSWER: It’s the same. A single car hitting a wall at 50 mph is the same as two cars hitting each other head on, each going 50 mph. From the point of view of a person in the car, it doesn’t make any difference if your car hits that wall or that your car hits another car coming at you. That is assuming both cars have the same mass. Of course, in the case of a head on collision with another car, there are now two vehicles that are a mess, instead of one.

The answer is counter-intuitive and seems to defy common sense. It would seem that having your 50 mph car hit another 50 mph car head on would be worse. But think about just your car. It must come to a stop by taking all of the kinetic energy (energy of motion) and turning it into heat and sound energy, either by hitting a car or hitting a wall. You can calculate the kinetic energy by the formula KE= ½ mv2, where KE is the kinetic energy, m is the mass and v is the velocity (speed).

But the other guy has to bring his car to a halt by either hitting the wall or hitting your car. All of his kinetic energy is used to bring his car to a halt. Each person is going to use the energy of motion to stop his car. You don’t borrow the kinetic energy from the other car.

You might try this. Suspend a ball on the end of a string from the ceiling. Pull it back, and let the ball hit the wall. It will bounce back a certain amount, which depends of the ‘bounciness’ of the ball (coefficient of restitution).

Now, if you let the ball hit another ball coming at it, which is also suspended on a string, you would find that your ball bounces back the same amount as it did when it hit the wall. This assumes the same kind and size of ball and wall is solid. It may be safer to do the “2 ball” experiment rather than use real cars!!

There is also Newton’s Third Law of Action Reaction in play here. For every action there is an equal and opposite reaction. One car hits another car, the other car pushes back.

Using real cars was shown on Episode 2 of Season 8 of Mythbusters a few years ago. They shattered the myth that two cars running into each other at 50 mph is the same as a single car hitting a wall at 100 mph.

On the show, they tested the myth by using scale models. They had pistons rigged up with cylinders of modeling clay. The piston, swinging at the end of a pendulum, was dropped from a height against a solid immovable object, a 1,200-pound steel wall. The clay was smashed a distance D. Then the piston was dropped from a height of twice the distance. The clay was mushed to 2D. These tests correspond to speeds of 50 mph and 100 mph collision against a fixed wall.

Next, a pendulum was matched up to an identical opposing pendulum. They were dropped simultaneously from the same height. The clay cylinder was found to squash down to the same amount as it did when a single pendulum hit the steel wall.

Then, of course, they did the experiment with real cars. They drove a car into a solid wall at 50 mph, then an identical car at 100 mph. Finally, they had two cars run into each other, both going 50 mph. Both cars were crushed the same as a car hitting the wall at 50 mph. The crumbling was the same. Neither experienced a 100 mph collision.

Send questions and comments to: lscheckel@charter.net.

Larry Scheckel is a retired Tomah High School physics teacher.

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