Is it possible for you to take someone with more experience to the scene to have a look around? There must be some kind of evidence. Large objects just don't crash into each other without leaving some evidence.

Yes, it's still possible to determine a rough speed considering that crashes like this are closed systems with some finite bound of momentum. You know one vehicle's momentum before the crash, the rest is just deductive reasoning.

Remember, the issue here isn't the weight differential between the two cars, it's the momentum differential.

Since he's describing - at least my understanding of it anyway - a very small crash scene, there are several reasonable assumption which can be made and then weeded through.  You can pretty much narrow the system down to either of the following:

1.) one vehicle was exceptionally soft, and thus had a very large ability to absorb the kinetic energy of the other, or
2.) the vehicles did not have a very large difference in momentum.

If 1 is true, then the situation becomes quite difficult.  If 2 is true, then the complexity is greatly reduced because he has an initial velocity for one of the vehicles.  I don't happen to think that 1 is true because of the damage he described.  2 is a far more viable candidate.

Of course, without seeing the scene, I can't get specific about much of anything.

Ashman, Not sure how the idea that the vehicles would have a similar momentum if mass of one vehicle is twice the other. To have similar momentum ( assuming momentum = mass x velocity) one veh would have to be travelling at a considerably differing speed.

Notenough... initial para says that the car is travelling at about 55mph, there are several comments about estiates of speeds etc. what sort of speed do you think the Yukon was doing? Was it about half of the Toyota?

I guess I have to respond to two people in one reply.  Interestingly, and notably, I'm happy to see that an actual case is again on the forum.  Thank you for that.

I can't literally speak, Blue, to what is you think and why; what you're capable of imagining or not.  But, I think where your confusion stems from is the assumption that both vehicles in this collision were traveling at the same speed.  Based on OP's original data, there is no good reason to think that both vehicles were traveling at the same speed, while there are good reasons to conclude that the momentum of both vehicles were close in magnitude.

You aptly assume the correct notion of momentum. As I pointed in 2 above, the vehicles likely had similar quantities of momentum.  Since the masses of the vehicles are known, and the velocity of the Toyota is known, we can, by deductive reasoning, conclude that the issue on point is the velocity of Yukon.

While I'm pleased that you agreed with me in times past, long before we knew one another, I'm disheartened that this is no longer true.  It is a first principle of science that physical processes leave behind evidence of their happenings.  That we may miss them isn't dispositive.  That we fail to apprehend them doesn't mean they aren't there; it merely speaks to our limitations, not to the theories of physics.

I am most disheartened that you've been called a liar because it's poor form to ascribe to mendacity that which can be described by ignorance.  Indeed, it flies in the face of science to posit more entities than are necessary, which this allegation of chicanery most assuredly does.  I do not doubt for a moment that you are sincere in your view that no physical evidence was left.  I am equally sure that the evidence is, or rather was (but no longer may be), there.  The trick is observing it, or inductively finding it.  In the absence of obvious forms of evidence, induction becomes much more difficult.  But there is no credible doubt that all physical processes leave behind evidence of their happening.  For it to be otherwise would indeed invalidate all of science.

I think it's good to get more minds and senses involved as this makes finding evidence more likely, particularly in hard to find type cases.  However, science isn't a democracy.  There isn't a majority vote.  There isn't strength in numbers (of the people who believe something).  In short, just because a couple of people conclude that they have found no evidence it doesn't follow that there actually isn't any. 

I look forward to what you have for us on next week.

I also think that 45 miles per hour is a little bit too high if this is a perfectly in-line type of collision.  The masses of the vehicles involved are very nearly in a 2:1 ratio.  In an in-line type of collision, for the momentums to be equal, we'd expect that the speeds would live very nearly in a 1:2 ratio so that the sum of the entire system comes out very nearly 1.  (That is to say that since the mass of Yukan is twice that of Toyota, the speed of Yukon would have to be half that of Toyota so that the momentums are very nearly equal).  Of course, this is probably not perfectly in-line, so your mileage will vary based upon what you have before you.



-- Edited by ashman165 on Friday 15th of May 2009 02:51:21 PM

BlueB wrote:

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Notenough, if the vehicles are travelling at simialr speeds as the estimated 55mph for both vehicles is later tweaked to extend to about 45-60 between them, their momentum remains roughly 2:1 unless you go to the extremes.

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This is categorically false.

 

I actually had a longer reply typed up but debating posting it for a while.  I finally settled on just a short, albeit highly accurate, response. 

I don't know how you look at things, and maybe this is just a personality thing between us.  We could simply have different ways of looking at things.  That said, with respect to the weights he gave us, as contrasted against the speed he gave us, there's no good reason to assume that he's giving us an estimate.  He seems bright enough to be able to tell us when something's an estimate as evidenced by his having told us that the speed for the Toyota was an estimate.  More than that, he told us how the estimated speed was come up with. 

I'm sure you'll agree that I tend to dissect things quite frequently.  I do it to those things which require analysis.  But in situations likes these, I don't presume to know more about what he's done and hasn't done.  I accept the data he gives me as though it was properly gathered, particularly when it's trivially doable.  Then I turn my attention to the questions he actually presents.  I guess at the root of it, it's simply that I don't presume to know more about what he wants to know than he himself tells me he wants to know unless there's some obvious reason to suspect a completely wayward mindset.  He has given me no such indication and I thus decline the invitation to automatically assume that he's incompetent and just came up with these weights all willy-nilly.

We do know that his speed for the Toyota is an estimate.  So far, based on what I've seen, it's the best information for the speed we have to work from.  So, I'm going to, ya know, use what's there.  He asserts that the speed was 55.  So, it might be 57, or 60, or 52. 

Anyway, what I was saying is categorically false is the same thing you repeated here:  "and [at] similar speeds their momentum [sic] will remain at about 2:1 . . . and its [sic] not until the speed differential heads to the extremes that it makes much difference." (Emphasis added)  That's simply untrue.  In this case, the thing at issue is that the vehicles seem to have roughly equal momenta.  We know that their masses are in a 2:1 ratio.

This means that at similar speeds, as you correctly noted, their momenta will be very nearly in a 2:1 ratio as well.  However, that doesn't imply that it will take extreme differences in speeds to make much of a difference.  indeed, for the two momenta to become roughly equal, we'll only need a difference in speed by a half factor (or double factor depending on your frame of reference).  That's hardly extreme.

It isn't until we get a differential in the momenta by a factor of 10 that things start to get suspect.  One fifth of that isn't enough to warrant it being an extreme. 

All of that presumes though that their momenta are equal, which isn't what we have going on here.  They are simply roughly equal, or at most, slightly less than equal. So, we're talking about something very nearly 1:1.  In this case, we might have .85:1, or 1:1.12.  Of course, we also have to consider a key detail he brought up:  that the collision isn't perfectly in-line.  So, there are angular considerations which must be taken into account.  But I don't have angles to deal with, so I'm dealing with what he's given me based on the laws of physics and principles of mathematics as well as I can.

I don't know about you, but to me 60 is pretty close to 55.  So close in fact that when one rounds 55, it becomes 60.  Granted, 60 is further from the estimate of 55 than 55.0000000000000001, but it is still quite reasonably close.   It's what? like 8% more?

And this is why I just decided on a short reply. =P


-- Edited by ashman165 on Monday 18th of May 2009 06:00:22 AM