Physics Lesson

[DrPizza]

Okay... for the educational part of this:
If 2 identical sized/weight cars travelling at 60mph crash head on, the impact on either car is the SAME as if it were to run into a tree (or solid brick wall, or whatever)

Since some of you are already shaking your head and saying "no, any idiot knows that it's like hitting a wall at 120 mph," let me attempt to convince you otherwise (since I'm a physics teacher, just saying so ought to be enough.)

First of all... the total energy involved in the collision is doubled... but there are 2 cars, so it's divided between the cars... Let's also try this: imagine holding a sheet up so that the driver can't see what he's hitting... whether it's a brick wall or a car travelling toward him... the sheet will remain more or less in the same place - unmoving (except for the various deformations that'll wrinkle it, body parts flying into the sheet, etc.)
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Anyway... Would *ANY* of you believe that someone would just get out of either car involved in a 115 mph head on collision with another car going 115 mph (standard equipped car, not Nascar with state of the art safety equipment, frames, multiple point harnesses, etc.) and walk away without a scratch?

Then why would you believe that someone could hit a tree and walk away? (in fact... unless you're ejected, you may not even be able to get out of the car without the jaws of life)

As promised, pics:
(these pics are all in the powerpoint that the state of florida is so nicely hosting)
pic1.jpg
pic2.jpg
pic3.jpg
pic4.jpg

powerpoint containing these (probably higher resolution): Here

Okay... now if someone survived... (let alone walked away from) a 115 mph crash, please provide a link to the news article. At least around here, a 115 mph wreck would have made it into the paper.

 

[Anubis]

these pics are really old, all the people in that crash died, obviously, i beleive they were drunk when it happened

and you can live through a crash at the speed, walk away? doubtfull,

 

[Kalvin00]

Repost from a long while ago, but pics are still shocking.

 

[DrPizza]

the pics are a repost... I added the physics lesson

 

[gururu]

the physics lesson is old too.

 

[James3shin]

DAYYYUUUMM!

 

[Mo0o]

Originally posted by: gururu
the physics lesson is old too.

yeah some guy named nuuwt0n posted that here YEARS ago

 

[pray4mojo]

Originally posted by: DrPizza
the pics are a repost... I added the physics lesson

On a somewhat related matter, would you like to assist me as I take Physics AP?

 

[Daedalus12]

Haha but you are wrong. If you are really a physics teacher then you shouldn't be teaching any physics class.

Link: http://www.physlink.com/Education/AskExperts/ae119.cfm

 

[m2kewl]

damn, those pics brings tears to my eyes...

 

[DrPizza]

Originally posted by: Mo0o

Originally posted by: gururu
the physics lesson is old too.

yeah some guy named nuuwt0n posted that here YEARS ago

Actually, come to think of it, I posted that lesson here years ago as well So, mine's a repost of a repost of my own

Originally posted by: pray4mojo

Originally posted by: DrPizza
the pics are a repost... I added the physics lesson

On a somewhat related matter, would you like to assist me as I take Physics AP?

The other night, I had actually typed a thread titled "~~**official**~~ math and science homework help thread", along the lines of CPA's official tax return help thread.

I put some simple instructions, including for other posters to provide "how to's" rather than just giving the answers and all the work. I figured I'd contribute a lot to the thread, and there are quite a few others who have consistently helped with various homework problems. (I remember one problem that was posted after some national test... it turned out the poster had posted an incorrect number - the total time spent on the problem by several of us probably added up to several hours... it was fun and challenging though)

Anyways.... I stared at the thread for a moment and thought: "what am I doing? am I insane? This'll actually encourage them to post even more problems" and I clicked cancel.

 

[DrPizza]

Originally posted by: Daedalus12
Haha but you are wrong. If you are really a physics teacher then you shouldn't be teaching any physics class.

Link: http://www.physlink.com/Education/AskExperts/ae119.cfm

Yes, you are correct... I usually specify that it's a solid wall, "strong enough to withstand the crash without getting damaged"... I had put the "solid" brick wall in my original post, but left out the rest out of laziness. (as opposed to a normal brick wall). But, even with an undamaged brick wall, it still absorbs some of the energy as thermal energy. Nonetheless, it is very small in comparison to the energy absorbed by the shearing, deforming metal of the car, especially since the cars are built to deform and absorb as much of the energy as possible.

In my class, I do a demonstration of kinetic energy being absorbed... The first time, I drop a 1 pound hardened steel ball onto the floor. (they're not going to be happy with me when they see all the dents one of these years). The floor is visibly deformed after the impact. I do the same onto a piece of paper laying on the floor. Again, the floor is deformed, as well as the paper.

However, I then take 2 one pound hardened steel balls and have everyone in the classroom hold up a piece of paper. I bang the balls together with the paper sandwiched between. It leaves holes and burn marks on the paper, and the odor of smoldering paper is very apparent. Since the balls do not deform very much (energy -> work), the kinetic energy is changed to thermal energy.

But, you're right. 2 cars colliding head on, each travelling 60 mph, is more like a car hitting head on, travelling 61 or 62 miles per hour. The most important concept is that the energy isn't doubled for the two cars. But yes... thermal losses are impossible to avoid. Even in the case where the paper doesn't have a hole and burn marks, there is still some loss of energy to heat.

 

[TuxDave]

Although I heard the 120mph argument, I never really put thought into it. But yeah, I agree, the amount of energy transferred to each car should be the same in the head on collision case and the brick wall case.

 

[myusername]

I would be very surprised if a straight head on collision bewteen any two modern "standard" equal weight vehicles traveling at 70mph each was survivable, much less 120.

Turning one of the objects into a brick wall would effectively increase the structural damage, considering the effectiveness of modern crumple zones. A tree would triple local forces on the car, which would have an unknown effect on the occupant .. depending on placement, the tree could cause more extensive cabin intrusion, or it might actually slow the rate of deceleration - only testing would show.

One of the most important factors for survivability has nothing to do with how damaged the structure of the car is, but the

forces that the occupants sustain during deceleration.

 

[desiplaya4life]

and you are old too

 

[Gurck]

First, have to point out some flawed logic - "First of all... the total energy involved in the collision is doubled... but there are 2 cars, so it's divided between the cars..." implies that in car vs. tree the car absorbs 100% of the energy - but this isn't the case, though granted a tree won't absorb as much as a car. I'm guessing ~30%, depending on various factors about the tree (age, girth, state of hydration, species, height of impact, etc).

Second, I've hit one at around that speed, though backwards. I walked away from it with nary a scratch. My car was destroyed. I got lucky, sure - but your claim isn't that it's unlikely, rather that it's impossible. I'm living proof.