# Car Crashes Again

1. Nov 22, 2005

### Shane

okay bringing up an old topic but

a car (lets just say 2000 kg) drives into a concrete wall (essentially immovable) at 40 km/h.
the same vehicle (make and model not the product of the colliision!) drives head on into another identical vehicle traveling in the opposite direction; both at 40 km/h.

Which does more damage?

They are equal no? Argue me if im wrong but also couldnt you argue that the vehicle crashing into the wall has momentum lost to recoil, rolling away from the wall after it has hit? All of course depending on the construction of the car right? those cars in demolition derbies are built (impracticably for the transfer of momentum and safety's sake) to withstand heavy blows, so couldnt that vehicle lose some momentum to recoil on colliding with the wall?

2. Nov 22, 2005

### Shane

Also quoting this website: http://www.sciam.com/askexpert_question.cfm?articleID=00042194-68C4-1C71-9EB7809EC588F2D7

"You will do less damage to your car and yourself by hitting a fixed object. The reason is that the damage done is a function of the relative velocity of the objects. If you are traveling at 35 miles per hour and hit a large tree, the relative velocity is 35 miles per hour. However, if you hit a car coming toward you at 35 miles per hour, the relative velocity is twice that. The effect of hitting the other car is therefore roughly the same as of driving into the tree at 70 miles per hour."

Is that true??? I had an extremely long argument with my physics teacher agreeing with the above quotation, but my physics teacher argued that even though the relative momentum is twice as much, there are two cars and the momentum is transferred as such with each vehicle taking half of the momentum during the crash.

3. Nov 22, 2005

### Mk

I swear we had this same thread twice in the last two months. Why don't you do a google search for it? Hell, I did it for you. https://www.physicsforums.com/showthread.php?t=97485 There's one.

Did you read the next two paragraphs of the Scientific American link. The next two said why your quoted explaination was wrong.

4. Nov 22, 2005

### ZapperZ

Staff Emeritus
Please do NOT start the same thread again! Respond to the existing thread that you have started already. Multiposting of the same topic is not allowed on PF.

Zz.