Two cars, each traveling at 18 miles per hour, one eastbound,one westound, collide with each other head on.
This collision creates the same amount of damage as one car, traveling at 25 miles per hour, hitting a parked car head on.
Two cars, each traveling at 18 miles per hour, one eastbound,one westound, collide with each other head on.
This collision creates the same amount of damage as one car, traveling at 25 miles per hour, hitting a parked car head on.
Why not equal to 36mph?
Streetwiseguy, double the speed will square the force
pilotbraden wrote: Streetwiseguy, double the speed will square the force
Square the energy, actually. Energy in the crash is relative to the area under the stress/ strain curve of the materials
Which direction was the car going 25 mph headed?
trucke wrote: Which direction was the car going 25 mph headed?
South, towards Albequerque. But in reality they were lost and someplace in Alabama.
trucke wrote: Which direction was the car going 25 mph headed?
Down.
Suppose a car crashes into a wall with a velocity v. While it is interacting with the wall, the wall exerts a force (F) on the car and the car exert a force F on the wall.
Where the force the wall exerts on the car and the car on the wall have the same magnitude. Now, what if I replace the wall with another identical car traveling at the same speed?
Since the initial momentums are the same and the forces are same, the effects are the same on the two cars. So two cars head-on are the same as one car into a wall.
-- quoted from scienceblogs.com/dotphysics
So the force of two cars colliding head-on at 18 mph is the same force of one car colliding into a fixed object at 18 mph. The only reason why hitting a parked car would result in less damage is that a parked car is not a fixed object--any distance it is pushed by the colliding vehicle is a transfer of energy away from the driven car.
In reply to nderwater:
Yes, I have had that explained to me as well, and it's tougher to grasp conceptually. At least for me. It helps to remember that the wall is infinitely vast, strong, and immovable. Much like this wall:
https://www.youtube.com/watch?v=RZjhxuhTmGk
In reply to volvoclearinghouse:
Holy crap that was cool!
But wait a minute: So 18mph into a wall is the SAME as two cars head on each traveling at 18mph?
Hungary Bill wrote: In reply to volvoclearinghouse: Holy crap that was cool! But wait a minute: So 18mph into a wall is the SAME as two cars head on each traveling at 18mph?
Or one car hitting a stationary one at 25 mph. Yeah. Basically, because the cars can deform, but the wall doesn't.
Now, imagine two cars hitting each other, but this time the cars cannot deform, but remain free to move. What happens than?
aha. So NOT taking deformation in to consideration, the following is true:
nderwater wrote: So the force of two cars colliding head-on at 18 mph is the same force of one car colliding into a fixed object at 18 mph. The only reason why hitting a parked car would result in less damage is that a parked car is *not* a fixed object--any distance it is pushed by the colliding vehicle is a transfer of energy away from the driven car.
But then again, how do you quantify "deformation"? It seems that my Impala would deform less than my Honda, and I can tell you for sure than my Impala does not deform when it meets the back end of a Renault Thalia (but the Thalia will deform enough that the trunk wont open).
Hungary Bill wrote: aha. So NOT taking deformation in to consideration, the following is true:nderwater wrote: So the force of two cars colliding head-on at 18 mph is the same force of one car colliding into a fixed object at 18 mph. The only reason why hitting a parked car would result in less damage is that a parked car is *not* a fixed object--any distance it is pushed by the colliding vehicle is a transfer of energy away from the driven car.But then again, how do you quantify "deformation"? It seems that my Impala would deform less than my Honda, and I can tell you for sure than my Impala does not deform when it meets the back end of a Renault Thalia (but the Thalia will deform enough that the trunk wont open).
Sum of the energies, I think. And what happens to the energy. In a crash, not all the energy goes into deforming stuff. Some goes into heat, some goes into noise, and some goes into other movement. But in all cases from two identical cars colliding, all the way on that continuum to a car hitting an immovable wall, the sum of the energies has to be the same.
Also, consider two billiards balls hitting each other square on. ore one of those things with the 5 balls suspended from strings.
Mythbusters did a whole episode on this. To make my comment even more relevant, here is a picture of Kari Byron with a .50 caliber rifle.
Elastic vs inelastic collisions. In a theoretical situation where the cars are inelastic, two cars hitting at 18mph is equivalent to car hitting wall at 36mph. But real life is elastic collisions and deformation adds all sorts of stuff to the equations :D
Hungary Bill wrote: In reply to volvoclearinghouse: Holy crap that was cool! But wait a minute: So 18mph into a wall is the SAME as two cars head on each traveling at 18mph?
KE=1/2 * M * V^2 is the formula you're after.
jsquared wrote: Elastic vs inelastic collisions. In a theoretical situation where the cars are inelastic, two cars hitting at 18mph is equivalent to car hitting wall at 36mph.
How is momentum conserved in an inelastic collision against a fixed object?
Hungary Bill wrote: In reply to volvoclearinghouse: Holy crap that was cool! But wait a minute: So 18mph into a wall is the SAME as two cars head on each traveling at 18mph?
Unless you're between them.
A train leaves Boston for New York at 4:00 PM and averages 60 mph. A second train leaves New York for Boston on the same track at 3:00 PM and averages 50 mph. When the trains meet is it better or worse than them hitting a wall?
ooh! I know this one: Electric trains don't emit smoke
What happens if two brick walls crash into each other? 
the mythbusters episode on this exact thing was pretty cool.. actually, they kinda sorta did it twice: in one, they rammed some Hyundais or something into each other head on with each going something like 60 mph... in the other, they rammed 18 wheelers together with each going something like 60 mph, with a Hyundai or something in between just to see what would happen to it..
stuart in mn wrote: What happens if two brick walls crash into each other?
An interesting parallel would possibly be billiards balls, or one of those swinging ball things. Which reminds me of a related question. You've seen the 5 balls on strings thing, right? (EDIT: I now see this is known as "Newton's Cradle") One of these:
So, when you pull one ball back, one ball on the other side pinks off. But if you pull two balls, it also pinks off two balls on the other side. And, if you've ever done 3 from one side, it plinks off 3 from the other side- effective meaning the middle ball of 5 completely changes directions. 'Splain this!
stuart in mn wrote: What happens if two brick walls crash into each other?
Have you ever divided by zero?
Don't do it man, the results can be unexpected. 
You'll need to log in to post.
