My friend says if you’re in a car traveling 60 mph and you collide head-on with a similar car traveling 60 mph, you feel the same impact as you would hitting a concrete wall at 120 mph. I say you feel the same impact as hitting the wall at 60 mph. Who’s right?

—Jim Selby, Henderson, Nevada

You are. In a head-on collision, the sum of the automobile speeds does not equal the force of the impact on each vehicle. Consider your example of a collision between identical cars traveling at 60 mph. They cannot each receive a 120-mph impact—in opposite directions! Instead, they share the combined “120-mph crash.” And as their masses and speeds are the same, they share it equally: Each vehicle receives a 60-mph impact.

The effect on each one is roughly equivalent to a crash in which a car runs into a wall of such mass that it doesn’t budge. The wall’s share of the impact is minimal; the vehicle bears virtually all of it.

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