Crash Talk Here

By the way, my limited mind thinks that the 2 cars would still be headed down the road at 12.5 mph whether they were made of down, or of granite.


[Okay, actually I think in the granite case you would get an elastic collision and car number 1 would stop and send car number 2 down the road at 25mph, but I'm talking about inelastic which I would consider a car crash to be.]

 

Hmmm.. EDIT - Nevermind. Deleted some blather about elastic and inelastic. Seems I'm on a roll - confusing terminology. :blink:

 

:lol: B)

 

Not taking any sides in this jihad, but I find the language of "absorbing energy" a little vague. It's just not a scientific or engineering concept. To refresh your views on what the definition of energy is, read the Wikipedia entry about energy.

I think what is happening here is that the kinetic energy of the moving car does some "work" by rearranging the state of the car at impact, and probably releases some heat as well. I don't see the energy content of the steel slab or granite wall changing much, unless it also heats as a result of the collision. :blink:

Please, DON'T try this at home!

Oh, and for the "heavier is safer" crowd, I ask you to perform this thought experiment with an Indy car (very light) to a Cadillac Escalade (very heavy): If each vehicle was traveling at 200 mph and hit the wall at the Brickyard, how would the driver fare?

 

If you hit the wall at the brickyard... then the car with the better crash rating is likely to have the best results. Usually (but not always) it is the lighter cars that have the best crash ratings.

It is much more common to have a collision between two vehicles. In those cases... the higher mass is usually more important than the crash rating results... but better crash rating results do help.

/Jim

 

Now theres an Idea.. does anyone want to settle for this once and for all and donate your car for a test and be a willing driver?

SloopG..... I don't think anyone got your hint anymore than they got the crash figured out!

Sorry for trashing your thread Sloop.......

If your still serious about a Camry... see if they are having a sale..... On the 19th they are having one for 2800 under msrp here.

 

windstrings, let me try to address your main misconception:

You make the claim that two objects that collide absorb the same amount of energy from the collision, no matter what material they are made of. I will attempt to prove this claim false using the following thought experiment:

Consider two identical lead balls attached to a spring. At time 1, you push the lead balls together such that the spring has potential energy 2E. You then let go of the balls. At time 2, the exact moment the spring reaches its resting length, the balls each have kinetic energy E. Then, the spring will stretch out to recapture the kinetic energy of the balls so that at time 3, the balls are stationary and the spring again has potential energy 2E. The spring then pulls the balls back in so that at time 4, the spring is at resting length and the balls each have kinetic energy E, but this time they are rushing inwards. The balls then compress the spring until they are stationary and the spring again has potential energy 2E as in time 1. The balls will continue to oscillate indefinitely with the energy being converted between kinetic and potential energy forever.

Now consider a different scenario: one lead ball with a spring attached to it, and a lead ball with no attachments. You fire the two lead balls at each other so that each has kinetic energy E, and such that the spring on the first is pointing directly at the second. When the spring first touches the second lead ball, you get a situation like time 4 above. The spring will then compress to the point where the balls aren't moving and the spring has potential energy 2E. Note that at this point in time the spring has captured all of the energy in the system. The lead ball with no attachment did not absorb any energy at all.

You can prove this by snatching away the ball with no attachments at the exact moment the balls stop, and replacing it with a third lead ball. The spring releases pushing each ball so that each has kinetic energy E. You've now recovered all 2E of the energy of the original system. If the first lead ball had absorbed energy, you'd in effect be creating energy for free, which is impossible.

Now replace the spring with a "one-way spring", i.e. one that compresses, but deforms permanently and doesn't expand. The ball with the spring is like a Prius: it absorbed all of the energy from the collision in the deformation of its structure. The ball without the spring is like the granite wall. Its kinetic energy was absorbed entirely by the Prius. In other words, the material and structure of the objects that collide determine fully the proportion of energy absorbed by each object. If both balls had identical springs, each would deform symmetrically absorbing equal amounts of energy.

I am a little curious how you're so sure you're right. As far as I can tell, you're only basing your arguments on intuition, which can be very misleading. Consider that at one point in time, people were certain that the sun went around the earth because it made no intuitive sense that it was the earth going around the sun.

 

Okay, I've been repeating the Conservation of Momentum law over and over, I'll start trying to use the Kinetic Energy argument.


Kinetic Energy is 1/2 * mass * velocity squared.


Prius at 25mph has Kinetic Energy of 1/2 * 3000lbs * (25 * 25) = 937,500.

2 Priuses at 25mph have Kinetic Energy of 937,500 * 2 = 1,875,000.

Prius at 50mph has Kinetic Energy of 1/2 * 3000lbs * (50 * 50) = 3,750,000.


A. Run 1 Prius at 25 into wall. Dissipate 937,500 of KE into 1 Crumple zone and a scratch on the wall.

B. Run 2 Prii at 25 into each other. Dissipate 1,875,000 of KE into 2 Crumple zones.

C. Run 1 Prius at 50 into wall. Dissipate 3,750,000 of KE into 1 Crumple zone and a scratch on the wall.


At the end of each crash, KE is 0, Nothing is moving.


A and B are equal. C is much worse.

 

D. Run 1 Prius at 50 into a Parked Prius. Had 3,750,000 of KE, Ended up with the 1,875,000. Dissipate 1,875,000 of KE into 2 Crumple zones.

D is also equal to A and B.

 

Run one Neon into one F250 powerstroke diesel, dissipate one neon into one towhook on the F250. Drive the 250 home, sweep the neon into the garbage.

Did that, got the tshirt. In fact, I crashed head-on with a neon with the f250 traveling about 10 mph and the neon probably 40 to 50mph.

The ford sustained a bent bumper, slightly bent tow hook (mounted to the frame). My seatbelt really didn't even tighten up and my airbags didn't go off.

The neon had the front shaved with a big V all the way to the front window. The top of all the motor junk was shaved off by my tow hook.

I'd suggest both vehicles absorbed half the energy here. Just that the crumple zone on the f250 was less crumply than the crumple zone on the neon. They showed differing amounts of visible damage in the end.

So, the wall does absorb half the energy. It just doesn't have the collapsable properties that the bending metal does.

Camery vs Prius... I wouldn't want to be in either. For this experiment give me a snow plow or my f250 and we'll try it.

 

From what I remember from physics (admittedly, it's been a while) the wall absorbs very little of the impact, as in C.

I took an after school physics class in high school, right across the street from the Daytona Beach racetrack. One of our questions in our AP essay practices involved taking the speed Dale E. hit the wall at and the distance of crumple in the car, and finding out what his deceleration was. I do remember she (best high school physics teacher in the country) asked us to 'assume the wall is undeformed.'

 

If a brick wall leaves Chicago traveling west at 45 mph, and a Prius (with NAV), leaves Los Angeles traveling East at 58 MPH, what route would result in the earliest collision, where would they collide, and what Toyota Dealership would be closest for towing purposes?

 

Please see my lead ball and spring analogy. The more deformable object will tend to absorb more energy than the less deformable object. In your case, the Neon's hood was more deformable than the F250's tow hook assembly.

 

Good point, had me for a minute, I started to see what windsprings was saying, and then I saw how it was wrong. They both start pushing, trying to get rid of the Kinetic energy, but the weaker material loses and takes most of it.

Put the wall on a soft spring that will allow it to convert all the kinetic energy to potential. Add some downy pillows so the Prius doesn't get damaged and all the kinetic energy will go into the spring. Does the spring help you visualize how the Kinetic energy gets split?

After all the kinetic energy is converted to potential, that spring is going to push that Prius back at the same speed in the opposite direction. Hybrid Technology at it's best. I think we should all have these walls in our garages.

 

Well SloopG.... looks like this thread is totally hijacked too!....

maybe you will have better luck asking questions about your 07 Camry at Freds House of pancakes! :blink:

There are just far too many genius's out here to merely talk about cars!

 

No, this thread is un-hijacked. Unfortunately for SloopG?

 

does having a higher mass necessarily mean having a greater ability to absorb more energy with crumple zones?

 

Plasm,

Now that we have established (finally) that two 3000 lb Prii in a head-on crash at 25 mph is the same as one Prius hitting a granite cliff at 25mph... I have a couple of questions that are not quite as intuitive to me.

1) From the perspective of the Prius driver... what is result of a head on crash with a different 3000 lb car that has inferior crash crumple zones? Would this be worse... or better than hitting another Prius.

2) From the perspective of the Prius driver... what is result of a head on crash with a 6000 lb car that has a crash crumple zone that can absorb the same amount of energy as the Prius? I do understand that the 6000 lb car would have a worse crash test rating than the Prius since its crumple zone would need to absorb twice the energy of the prius. What would be the result on the driver of the 6000 lb car?

/Jim

 

My 2 cents:
It is not only how much KE is disipated, but how much time it takes to disapate (an indirect measurement of G acceleration seen by the occupants).

When a 3,000 pound and 6,000 pound vehicle colide head on the occupants of the 6,000 pound vehicle will experience half the max G acceration of the other vehicle(assuming same crumple zones).

A piece of paper being blown toward my bumper at 5 mph will experice a very high G load when I hit it a 75mph. I won't feel anything. Similar to a train hitting a car at a railroad crossing. I'd rather be in the heavier vehicle.