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Palomar Observatory

Thursday, July 28, 2011

Crash Bang

I saw this letter in the local paper, the Village News this morning:

Re: Collision at Main and College intersection results in fatality [Village News, 7/14/2011]
If you were blindfolded and unfortunate enough to be in a car going 20 mph that crashed head on into an identical car going 20 mph, followed the next day by being in a car going 20 mph when it crashed into a brick wall, you would not be able to tell the difference. The forces involved on you would be identical. In each case you would come to an immediate stop and then bounce back a little.
The [CHP] should not add the rates as suggested in the article. If they don’t believe it, please ask another mathematician or physicist. If they still don’t believe it, they should refrain from voting in the next election.
Jerry Maurer
Now this letter sounded a trifle counterintuitive to me, the art and architecture major. Mr. Maurer would have us believe that there is no difference in the collision force between a sedentary wall and an oncoming car and two cars crashing into each other.

Unfortunately, Jerry is a neighbor of mine, a lot smarter than I am and a retired long time math teacher so I think that it would be prudent to take him at his word, no matter how incredible his postulation sounds.

But why does it sound so odd? Doesn't the energy from car #2 add anything to the equation? I wrote Jerry a little note this morning, which he has not yet responded to:
Hi Jerry,
Robert Sommers here. A bunch of us were reading your letter at the coffee shop and I am puzzled with the car crash explanation. If I throw a baseball, let's say an offspeed pitch, at a stationary bat it may only travel thirty feet after contact. Now if I throw a fastball at the bat and the bat is swinging, the ball is probably going over the fence. Force seems to be compounded. Why are the forces not compounded in the car scenario? What am I missing?
We were batting this around (no pun intended) and friend Bill sent me this explanation http://warp.povusers.org/grrr/collisionmath.html but the whole thing stills seems screwy. I guess that is why I was a liberal arts major...
           Best to you and your wife, hardly ever see you anymore.

           Robert Sommers

Can anybody shed some logic here and explain this arcane mystery to me?







3 comments:

Anonymous said...

I'm stll hung up on figuring out if you're in a car going at the speed of light, and you turn on the headlights, does anything happen?
And what does happen if an irresistible force meets an immovable object??
-E

Anonymous said...

Robert,

You Posted this:

"If you were blindfolded and unfortunate enough to be in a car going 20 mph that crashed head on into an identical car going 20 mph, followed the next day by being in a car going 20 mph when it crashed into a brick wall, you would not be able to tell the difference. The forces involved on you would be identical. In each case you would come to an immediate stop and then bounce back a little."

The reason that the first part is true is due to the statement "car going 20 mph that crashed head on into an identical car going 20 mph".

Since the vehicles are identical they have equal mass and equal deceleration (crush) zones. The rate of deceleration is what is in question here.

Each vehicle will crush at the same rate therefore each vehicle will stop in the same amount of time/distance, therein dissipating the energy at exactly the same rate.

Therefore the deceleration felt by the occupants would be identical.

When impacting the brick wall at 20 mph you have one crush zone to deal with and the forces would be identical to the two car incident as the deceleration time/distance is equal to the first example.

Note this is only true if both vehicles are identical in all ways including mass. If vehicle one has two 300 pound occupants and a full trunk of bricks and vehicle two has a 120 pound driver and nothing in the boot then vehicle two gets the short end of the stick.

One final thing, the statement "In each case you would come to an immediate stop" is erroneous. That would indicate a acceleration rate of Infinity. Not doable in this universe with any known materials. You always will get some deformation.

The only example of this I have ever seen contemplated is called the "Bergenhlom inertial-less drive system"..

See: http://en.wikipedia.org/wiki/Bergenholm_space_drive and http://en.wikipedia.org/wiki/Inertialess_drive

Later,

Hudgins

Blue Heron said...

Hey E, don't forget that at some point you'll be getting younger too. Hudgins, masterful, thank you, still sounds like bullshit though. We may have to raise some dough and field test this one.