Displacer on 8/10/2007 at 21:15
Gotta add my observations.
I'm not sure which happened so I'll go through both.
If it was a piece of tire kicked up from the road by the truck then the only force acting on the piece was the rotation of the tire. The piece was stationary on the road, and the forward speed of the car would not matter as we're dealing with the backward travel of the piece. The truck tire would ride up on top of the piece, at that point the rotation of the tire would act on the piece instead of the road. It would be propelled backwards (assuming friction of the piece on the road was overcome). The speed at which it was propelled would depend on the rotational speed of the tire.
If it was from a blowout, then you have another force acting on the piece that was broken off, which is the centrifugal force of the rotating tire, which is a force acting on all parts of the tire forcing it outward. Now the only forces that could propel the piece backward would be centrifugal force plus whatever force there was from the explosion of the tire. In order for the person to get hit at 120mph the piece would have to be propelled backwards at 120mph, it would have to overcome the 60mph forward velocity that its currently traveling at, and gain another 60mph in order to hit the guy at 120mph (60mph backwards speed of the piece + 60mph forward speed of the guy)
hopper on 8/10/2007 at 22:03
You are making the same mistake as Mazian, here. What you guys fail to realise is that the truck wheel does not exert any horizontal, i.e. backwards, force on the piece of debris. The truck is moving forward at 60, the part of the wheel touching the road at any one time is moving backwards at 60 relative to the truck - so the forward movement of the truck and the backward movement of the bottom of the wheel cancel each other out. Relative to the road, the bottom part of the wheel is not moving, therefore you cannot add the rotational velocity of the wheel. It's irrelevant. The moment the truck is driving over the piece of debris, the only force effecting it is downwards, which means it will bounce up - straight up - when the wheel passes. If anything, the debris might be hurled somewhat forwards, because of surface tension and drag.
BTW, nice pics, heywood.
Pyrian on 8/10/2007 at 22:13
Heheh, ah, heywood, nice diagrams. It's true that some people here disagree with you solely because they haven't comprehended your argument and perhaps your diagram will help those people.
Thing is, you're still wrong.
First, the exploding tire (i.e., not the actual CSI situation, but it's getting discussed a lot, so...). The forces acting on an exploding tire have relatively little to do with the starting velocities you so meticulously diagrammed, nor with centripetal force as others have asserted (it's worth noting that centripetal force is little more than a convenient way of summing the forces you diagrammed - it should never conflict with the diagram because it's basically the same thing). Instead, the by-FAR strongest force acting on an exploding tire is its internal air pressure. This air pressure has to be high enough to support the entire weight of the vehicle in a relatively small area! That air pressure is easily - easily - enough to throw a small piece at high velocity.
I don't know why you ignore that component. It's much more significant than everything you diagrammed.
Of course, the CSI situation was not an exploding tire. Typically, when road stuff gets thrown into the air, it's stuff that got stuck in the tread or to the rubber of the tire when the tire was directly over it, then gets thrown off as the tire is rising. Such debris would be correctly characterized as generally travelling upward and frequently forward, though merely being thrown in the air is sometimes enough for a car behind to hit it. So, that action could not explain a fragment going backwards at high velocity, and for precisely the reasons heywood so kindly demonstrated.
However, there are other, less common possibilities that could account for that. Normally, rubber tires have very high traction (and friction) - that's how they're designed. So stuff sticks to them. But if something isn't sticking, a differential pressure can cause an effect that's basically the same as the tire exploding - i.e., a large amount of pressure on a small object resulting in high velocity. If the tire is entirely on top of the debris, then that cannot happen; it can only happen while the tire is only partially on top of the object, and when the object is not sticking to the tire and/or ground with more force (friction) than the net "wedge" pressure exerted on it. If there's oil on it, that could happen.
hopper on 8/10/2007 at 22:32
Basically, you're saying he's wrong, and then you continue to prove he's right. :cool:
I'm not sure I understand what you mean with your differential pressure example. Is it the same thing as "shooting" an orange seed from between your fingers by squeezing it?
Shug on 8/10/2007 at 23:09
Quote Posted by theBlackman
Good visuals. :thumb: :thumb:
I, for one, found the green racing car outrageously misplaced and immersion-breaking.
Verdict: CSI could have done it better
heywood on 8/10/2007 at 23:26
Quote Posted by Pyrian
First, the exploding tire (i.e., not the actual CSI situation, but it's getting discussed a lot, so...). The forces acting on an exploding tire have relatively little to do with the starting velocities you so meticulously diagrammed, nor with centripetal force as others have asserted (it's worth noting that centripetal force is little more than a convenient way of summing the forces you diagrammed - it should never conflict with the diagram because it's basically the same thing). Instead, the by-FAR strongest force acting on an exploding tire is its internal air pressure. This air pressure has to be high enough to support the entire weight of the vehicle in a relatively small area! That air pressure is easily -
easily - enough to throw a small piece at high velocity.
I don't know why you ignore that component. It's much more significant than everything you diagrammed.
If you go back and read my posts, you'll see that I did acknowledge air pressure. Now here is why I don't think it's that significant. First, when the tire explodes, it deflates extremely rapidly. With nothing like a gun barrel to constrain it, the air expands in all directions and the pressure drops almost instantly. So it is only pushing with enough force to accelerate the fragment significantly for a brief instant. I'd have to solve some integrals to give you an accurate estimate of the ejection speed, but when this thread started I made some assumptions, did some math, and came up with something like 40 mph. Second, and more importantly, the part of the tire that is moving fastest relative to the go-kart is the bottom, and the air pressure is pushing that down, so it won't be accelerated backward by air pressure. And the back of the tire would have to be accelerated by air pressure more than 60 mph backward just to equal the bottom of the tire. So, I'm convinced that the bottom of the tire would hit the go-kart first and hardest.
Quote:
However, there are other, less common possibilities that could account for that. Normally, rubber tires have very high traction (and friction) - that's how they're designed. So stuff sticks to them. But if something isn't sticking, a differential pressure can cause an effect that's basically the same as the tire exploding - i.e., a large amount of pressure on a small object resulting in high velocity. If the tire is entirely on top of the debris, then that cannot happen; it can only happen while the tire is only partially on top of the object, and when the object is not sticking to the tire and/or ground with more force (friction) than the net "wedge" pressure exerted on it. If there's oil on it, that
could happen.
Yeah, it could. The question is how fast could something be "wedged" out by a rolling tire? I don't even know how to estimate that, but I suspect it wouldn't be that fast.
EDIT: Pyrian & hopper: I'm glad we got back to discussing real factors again.
Displacer on 9/10/2007 at 00:33
Quote Posted by hopper
You are making the same mistake as Mazian, here. What you guys fail to realise is that the truck wheel does not exert any horizontal, i.e. backwards, force on the piece of debris.
I understand that, my point was when the tire is on the piece, it could propel the piece backwards
only if it overcame the friction of the piece against the road. Its the same principle as spitting gravel, you overcome the friction between the rocks and the ground, the tire spins faster propelling the rocks backwards. it doesn't matter what speed you are traveling when this happens.
37637598 on 9/10/2007 at 01:47
Quote Posted by Shug
I, for one, found the green racing car outrageously misplaced and immersion-breaking.
Verdict: CSI could have done it better
Not to mention Racist in every way possible!
Microwave Oven on 9/10/2007 at 02:32
Ok, so what I gather from this thread, I was right, my friend was as wrong as CSI, and thanks to heywood I have a nice diagram to shove at them.
Thanks everybody for the discussion.
dvrabel on 9/10/2007 at 08:36
Quote Posted by Mortal Monkey
What gives you the highest chance of survival, a head-on collision with a big tree at 40 MPH, or a 40 MPH head-on collision with a completely identical car going the same speed in the opposite direction?
In the head on collision both cars are can be considered to be each car separately colliding with a stationary and perfectly rigid object; the kinetic energy of each car is dissipated solely in that car's crumble zone.
The tree is not a perfectly rigid object and will dissipate some energy in the collision. Therefore, you're (marginally) more likely to survive crashing into the tree.
