Impact in collisions

A feature called Ask Marilyn in Parade magazine last Sunday contained the following statement dealing with some previous correspondence on the subject of impacts in collisions:

"In a head-on collision, the sum of the vehicles' speeds does not equal the force of the impact. This is a common misconception, sometimes propagated by news reports. Say two identical cars are traveling at 50 mph and they collide head-on. Each car sustains a 50 [-] mph impact, not a 100 [-] mph impact."

I didn't study physics, but to me this seems profoundly counter-intuitive, and in reports of head-on train collisions we often read about the combined speed at the time of collision, yet the magazine feature (which has committed other factual errors in the past) seems to be saying that the combined speed is irrelevant. Specifically, it says that "a head-on collision between a 15 [-] mph bike and a 35 [-] mph car will not deliver a 50 [-] mph impact to either party."

Yet it's the same collision. Would someone who is up to speed on physics care to comment?

Try this:

http://warp.povusers.org/grrr/collisionmath.html

The Mythbusters tested and proved this ...

They took identical make/model cars, and whammed one into a wall at 50mph, then another into a wall at 100mph.

Then they took two of them and whammed them into each other head-on each traveling at 50mph.

The impact damage was almost identical to the 50mph wall tests.

So if a car was going 50 mph and crashed into a stone wall that was doing 50 mph in the opposite direction, THAT would be worse than a 50mph crash into a stationary stone wall?

Ocala Mike wrote:Try this:

http://warp.povusers.org/grrr/collisionmath.html

Thanks for the answer. I never thought I had a head for the physical sciences, and I was right. It seems to be like the time when everybody "knew" that a big rock will fall faster than a small rock, until Galileo dropped one of each from the Leaning Tower of Pisa at the same time, and they hit the ground at the same time.

This reminds me of a conversation that was in "Trains" magazine many years ago. It referred to a tug-of-war between two Mallet locomotives, I think. The locomotives were shoving against each other. The article said that no drawbar could withstand the combined force of the two engines if they were both pulling in opposite directions.

It wasn't long before someone wrote in and reminded them that the tension on the drawbar could not be greater than if they were pulling against a coupler attached to a fixed (very well fixed, in this case) object. Once you slip your drivers, game over.

Les

I remember reading that article years ago. A similar photo showed a pushing contest between a modern steam locomotive and an electric locomotive. I can't remember the caption but the gist was the steamer got the jump on the motor but once the throttle was opened on the motor it brought the steamer to a halt and began to push it off the stage! Lots of smoke and noise, but in the end it just couldn't fight progress.