Straight Dope Message Board > Main Head-on Collision Physics - revisited
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#1
08-04-2004, 12:34 AM
 astro Member Join Date: Jul 1999 Location: Taint of creation Posts: 28,364

I had some time this evening so I revisited an old thread I'd been meaning to review for some time,

Did we ever some to some final and absolute conclusion in this thread as to the OP's question? Was he correct or was his friend correct re the physics of the collision? I'm assuming per the final comments that the OP was correct and his "knucklehead" friend was wrong.

Quote:
 I have a knucklehead friend I am unable to convince of a fairly simple physics problem. He contends if two cars of equal mass (discounting crumple zones and the like) both traveling 35 mph meet head-on, the resulting collision is multiplied to a 70 mph crash. I told him it's the speed of stopping that's important, that this scenario is no different than had one car traveling 35 mph run into a immovable brick wall --- the car goes from 35 to zero near-instantaneously. He disagrees, in the face of logic, and I can suffer his foolishness no longer. I'm long past my days of high school physics, and can't quote the various laws which lay waste to his folly. Any takers?
#2
08-04-2004, 12:58 AM
 Richard Pearse Member Join Date: Aug 2001 Location: Australia Posts: 7,456
It would seem to me that the effect would be the same as a 70mph crash between two vehicles, one stationary. It would probably also be the same as a 35mph crash into an immovable wall (half the speed but also only half the impact absorbtion). So they are both right, they just not talking about the same thing.
#3
08-04-2004, 01:03 AM
 SPOOFE Guest Join Date: Apr 2000
When two objects collide, the kinetic energy of one object is imparted onto the other object.

A stationary object has no (appreciable) kinetic energy. A moving object has plenty of the stuff, and will be only too happy to transfer it to its target.

Quote:
 this scenario is no different than had one car traveling 35 mph run into a immovable brick wall --- the car goes from 35 to zero near-instantaneously.
The key word is "near-instantaneously". To our eyes, two 35-mph objects would decelerate about as fast as a 35-mph object hitting a stationary object. However, if you actually got down to the nitty-gritty and measured things in teensy fractions of seconds, you'd probably find that the two moving cars decelerate even MORE, uh, near-instantaneously.

The collision would send a shockwave through the vehicle... it's not just felt at the front end, at the point of impact, but throughout the whole thing.
#4
08-04-2004, 01:39 AM
 Chronos Charter Member Join Date: Jan 2000 Location: The Land of Cleves Posts: 47,975
Two cars hitting head-on at 35 MPH is equivalent to one car at 70 MPH hitting a parked car, or to one car at 35 MPH hitting a much more massive object (the proverbial brick wall). Or, for that matter, a car going 140 rear-ending a car going 70. Or a car going 70 rear-ending a brick wall going 35.
#5
08-04-2004, 01:48 AM
 SPOOFE Guest Join Date: Apr 2000
Here's another way to think of things:

Energy needs to come from somewhere, and it needs to go somewhere. In the two-car collision, we have enough energy to get two cars up to speed. In the car-hit-wall collision, we have enough energy to get one car up to speed.

Twice as much energy means twice as much energy needs to go somewhere. It can't just vanish into the cosmos (well, some of it does, in the form of heat, but that wouldn't account for all of it).
#6
08-04-2004, 07:16 AM
 minor7flat5 Charter Member Join Date: Sep 2002 Location: Trenton, NJ Posts: 3,356
How about if we consider a perfectly ideal experiment where we smash two mirror-image cars together, each going 35mph.

Since the experiment is ideal, wherever there is a protrusion on car A, there is a matching protrusion on car B. Consequently, there would be no reason for any part of car A to extend past any part of car B. It would be as if there were an invisible plane separating the cars.

If you had a sufficiently durable and thin material, such as Lexan, you could place a sheet of the stuff at the precise point where the invisible plane would be, and the two cars could smash into it, leaving it standing. Now what's the difference between smashing into your sheet of Lexan while it is backed against a wall and smashing into it when there is a compensating smash coming from the other side? The divider never moves.
#7
08-04-2004, 07:17 AM
 minor7flat5 Charter Member Join Date: Sep 2002 Location: Trenton, NJ Posts: 3,356
Quote:
 Originally Posted by SPOOFE Twice as much energy means twice as much energy needs to go somewhere.
There's twice as much steel crumpling, absorbing twice as much energy. Each car is still crumpling to the tune of 35mph.
#8
08-04-2004, 11:54 AM
 Strainger Guest Join Date: Apr 1999
I've taken the file I talked about here, converted it to PDF, and posted it here (WARNING: PDF, in case you didn't notice that the first time). Don't get too caught up in the "Toyota Camry" thing; it's a joke.
#9
08-04-2004, 12:21 PM
 SCSimmons Guest Join Date: Mar 2001
Quote:
 Originally Posted by Chronos Or a car going 70 rear-ending a brick wall going 35.
And just try explaining that one to the insurance company!
__________________
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"You won't like me when I'm angry. Because I always back up my rage with facts and documented sources." -- The Credible Hulk
#10
08-04-2004, 01:50 PM
 Absolute There are no absolutes. Charter Member Join Date: Apr 2000 Location: In flight Posts: 3,669
Wait a minute: energy increases with the square of velocity, right? So two cars hitting each other at 35 mph is not an equivalent impact to one car hitting a stationary car at 70. 2 * 35 ^ 2 < 70^2 - a car moving at 70 mph has more energy than two cars moving at 35.

Or am I missing something...
#11
08-04-2004, 04:20 PM
 ZenBeam Charter Member Join Date: Oct 1999 Location: I'm right here! Posts: 6,896
Two cars at 35 have energy proportional to 2 * 35^2 prior to collision, and 0 afterwards.

One car at 70 and one at zero have 70^2 + 0 before collision, and 2 * 35^2 after collision, and are both moving 35 MPH. They slow down and stop through friction with the ground, but that isn't nearly as destructive, and so is being neglected. So the collision itself disipates energy proportional to 2 * 35^2 in both cases.

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