theBlackman on 7/10/2007 at 08:42
You don't need a lubricant to cause something to be expelled from between two opposing forces.
In this case, as extreme as the premise is, is a fragment of a flexible material being compressed under the weight of the truck, and as the weight of the truck tire *a high friction material* comes off the fragment, and the fragment is recoiling from the roadbed by its own elasticity, and the "wedge" factor of the area of tire/road contact is allowing the tire scrap/fragment to be picked up by the rotating tire and expelled into the air behind the truck.
It happens all the time with fragments of wood, tire debris, gravel and other objects on the highways.
I've had windshields in automobiles fractured by such objects thrown back by the tire of a vehicle ahead of me. The combined speed of my vehicle and the force generated by the impetus of the "throw" (for lack of a better word) of the tire of the car ahead have been sufficient to cause a relatively light object to have speed enough to generate a impact force much greater than the normal mass of the object would be expected to have.
The physicists can give you angles of vector, mass vs. inertia and the speed that would a small mass a factor of X on impact.
CSI may be stretching it, but high impact from such objects ( a 5 inch wide, by 6 inch long, by 1/2 to 3/4 inch thick piece of rubber) can give you one hell of a smack if you hit it at 60 MPH and it is just bounced up off the roadbed, let alone thrown back at you after the tire ahead picks it up and kicks it back at you.
A soccer ball sure comes at you in a speed that is many factors of the running speed of the player who kicks it. An object kicked back by a tire can do the same.
If you've every seen gravel kicked back by a spinning tire, you know what I mean.
Combined impact speed of two objects on direct 180 reciprical courses has always been A Plus B.
ffox on 7/10/2007 at 08:52
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?
Have you seen the (
http://www.youtube.com/watch?v=k3ygYUYia9I) video of the little car mangling the big car?
The little car is smashed backwards by the hit but the big car driver is the one who would have come off worse.
heywood on 7/10/2007 at 12:26
Quote Posted by theBlackman
You don't need a lubricant to cause something to be expelled from between two opposing forces.
In this case, as extreme as the premise is, is a fragment of a flexible material being compressed under the weight of the truck, and as the weight of the truck tire *a high friction material* comes off the fragment, and the fragment is recoiling from the roadbed by its own elasticity, and the "wedge" factor of the area of tire/road contact is allowing the tire scrap/fragment to be picked up by the rotating tire and expelled into the air behind the truck.
It happens all the time with fragments of wood, tire debris, gravel and other objects on the highways.
After being run over by a tire, that stuff is almost always thrown up and not backwards. Be it gravel, rainwater, dust, debris, or whatever. Remember, no part of a rolling tire is ever going backwards.
It may be theoretically possible for the "wedge" effect to squeeze something backwards, but as Pyrian says it would have to be of just the right shape & material and possibly lubricated. And it probably doesn't apply in this case because the tire itself is self destructing and not rolling over some debris.
Quote:
CSI may be stretching it, but high impact from such objects ( a 5 inch wide, by 6 inch long, by 1/2 to 3/4 inch thick piece of rubber) can give you one hell of a smack if you hit it at 60 MPH and it is just bounced up off the roadbed, let alone thrown back at you after the tire ahead picks it up and kicks it back at you.
Indeed, getting hit with 10 or 20 lbs. of steel belted truck tire tread traveling at 60 mph would not be fun. It might not cause a clean decapitation, but I could see it killing somebody.
Quote:
A soccer ball sure comes at you in a speed that is many factors of the running speed of the player who kicks it. An object kicked back by a tire can do the same.
If you've every seen gravel kicked back by a spinning tire, you know what I mean.
The soccer ball is stuck by a foot which is traveling much faster than the player. There is nothing analogous to the kicking foot in this case.
And for the n-th time, the tire is not spinning relative to the road. It is rolling on the road, which means there no part of the tire has any backwards component of velocity.
rachel on 7/10/2007 at 13:18
Quote Posted by heywood
And it probably doesn't apply in this case because the tire itself is self destructing and not rolling over some debris.
I know I was tired but I thought they made it pretty clear that it did. The truck's tires themselves are fine all along, and it's a piece of debris that was already on the road that was projected backwards
Mortal Monkey on 7/10/2007 at 13:39
Quote Posted by ffox
The little car is smashed backwards by the hit but the big car driver is the one who would have come off worse.
Losing your legs is not quite so bad when the alternative is getting a ruptured aorta. Besides, that wasn't a clean head-on collision.
heywood on 7/10/2007 at 14:23
Quote Posted by raph
I know I was tired but I thought they made it pretty clear that it did. The truck's tires themselves are fine all along, and it's a piece of debris that was already on the road that was projected backwards
The OP said:
Quote:
So I was watching CSI last night with a friend, and the episode had a case where a guy racing in a gokart got decapitated by a chunk of tire that came off a semi trailer he was following.
And for the last time, tires that are
rolling forward don't throw stuff backwards. :mad:
Mazian on 7/10/2007 at 15:00
Quote Posted by heywood
After being run over by a tire, that stuff is almost always thrown up and not backwards. Be it gravel, rainwater, dust, debris, or whatever.
"Almost always" =! always. I commute on the interstate every day and I never follow semis because my windshield
always gets hit by a piece of gravel.
Quote Posted by heywood
Remember, no part of a rolling tire is ever going backwards.
The bottom half is
in relation to the trailing kart. If you cannot grasp this then it is beyond my capacity to explain it to you. If the bottom half of the tire wasn't moving backward relative to the kart how in holy hell do you think the material was ejected
backwards?!!
Quote Posted by heywood
Indeed, getting hit with 10 or 20 lbs. of steel belted truck tire tread traveling at 60 mph would not be fun. It might not cause a clean decapitation, but I could see it killing somebody.
The speed of the ejected material is independent of the truck speed. Its ejection speed is a function of the weight of the truck and the radius of the tire, period.
Quote Posted by heywood
The soccer ball is struck by a foot which is traveling much faster than the player. There is nothing analogous to the kicking foot in this case.
Actually it's not a bad analogy.
Quote Posted by heywood
And for the n-th time, the tire is not spinning relative to the road. It is rolling on the road, which means there no part of the tire has any backwards component of velocity.
I've never claimed such, the tire is spinning
in relation to the following kart. The road does not factor since it's, from a relativity standpoint, identical to a treadmill. It does not matter if the truck is "rolling" or spinning its wheels on a treadmill, from a
physics perspective (in the right system) they are identical.
The fact that you cannot grasp the concept of relative motion or keep your frames of reference straight suggests you ought to consult a physics textbook rather than posting ITT.
heywood on 7/10/2007 at 17:16
Arguing harder isn't going to help you. First you write:
Quote:
This is incorrect. The top half of the tire is always moving forward, and the bottom half is always moving backward (the road has nothing to do with it). In addition, the speed of the tire fragment (relative to the following go-kart) must be calculated by determining where on the tire the fragment came from, measuring the distance from the fragment to the center and figuring out the speed using the rpm's of this particular tire at 60 mph. You then convert this speed to mph to get the ideal case ejection speed.
Which is wrong because we don't need to worry about the distance of the fragment from the center or the rpm to figure out the max speed. We already know the speed of the bottom of the tire relative to the go-kart. It's 60 mph.
Then you write:
Quote Posted by Mazian
I meant that the top and bottom halves are moving toward and away from the kart. The entire truck is not moving at all with relation to the kart. If both vehicles have the same velocity, the road can be said to be acting on them equally and can be eliminated from the system. Unless there's something I'm forgetting; Physics class was a long time ago.
The radius of the tire determines its speed. The outer edge of a tractor tire traveling 60 mph down the road is traveling faster than the outer edge of a little Radio Flyer wagon wheel at the same speed, it's geometry.
Which is also wrong. The outer edge of a tractor tire traveling at 60 mph down the road is NOT traveling faster than the outer edge of a little Radio Flyer wagon wheel traveling at 60 mph down the road. They are both traveling at the same speed. The bottom of each tire is stationary with respect to the road and -60 mph relative to the go-kart. And the top of each tire is +120 mph relative to the road and +60 mph relative to the go-kart. The radius of the tire doesn't matter and neither does the rpm; since the speed is known, they cancel eachother. Think about it.
And then you write:
Quote:
The speed of the ejected material is independent of the truck speed. Its ejection speed is a function of the weight of the truck and the radius of the tire, period.
I'd love to see you explain how to calculate the speed based on the weight of the truck and the radius of the tire.
Quote:
I've never claimed such, the tire is spinning
in relation to the following kart. The road does not factor since it's, from a relativity standpoint, identical to a treadmill. It does not matter if the truck is "rolling" or spinning its wheels on a treadmill, from a
physics perspective (in the right system) they are identical.
The fact that you cannot grasp the concept of relative motion or keep your frames of reference straight suggests you ought to consult a physics textbook rather than posting ITT.
Believe me, I understand the different frames of reference. I thought we got past this 2 days ago. They are not identical because drag acts to slow the tire fragment after it separates from the tire. Both air drag and possibly road drag if the fragment bounces on the road. But they are equivalent enough to establish the same
maximum impact speed of 60 mph for a fragment leaving the tire.
You may not have noticed, but what we're discussing now is whether a tire that runs over an object can propel it backwards (relative to its previous state) so that the impact velocity with the kart would be greater than 60 mph. To put it in terms of your treadmill analogy: Suppose the go-kart is stationary on a treadmill that's moving at 60 mph and you stick a piece of debris on the treadmill surface. It will impact the kart at 60 mph. Now suppose the truck is stationary and in front of the kart and the piece of debris rolls under the truck's tire. Now, is there a mechanism through which the tire can accelerate the piece of debris and cause it to hit the kart at > 60 mph? That is the point we are discussing now.
hopper on 7/10/2007 at 17:57
Quote Posted by Mazian
The road does not factor since it's, from a relativity standpoint, identical to a treadmill. It does not matter if the truck is "rolling" or spinning its wheels on a treadmill, from a
physics perspective (in the right system) they are identical.
This is where you get your analogies mixed up. First you say the road need not be included into the system, then you liken it with a treadmill, which is sort of slipping it back in. You probably aren't aware of this yourself. If you use the treadmill as an anology, you've brought the road back into the equation.
Specifically in your treadmill example, this is where the "road" (ie. the band of the treadmill) matters, because if the cars are staying in the same position while the treadmill is moving at 60 mph, then that's where the piece of debris gets its speed from, not from the tire of the truck running over it. (Edit: I'm assuming the piece of debris is lying on the treadmill, moving with it, and being hurled upwards by the truck as it passes over.) The bottom of the truck's wheel
is not moving relative to the treadmill, so any horizontal (backwards) velocity the piece of debris would have, would come from the treadmill, not from the truck's tires. The truck tire itself does not give the piece of debris any horizontal impulse, unless it's skidding (in which case the piece would be dragged forwards) or doing a burnout (which would propel it backwards).
SubJeff on 7/10/2007 at 19:14
Quote Posted by heywood
The head-on collision. That one's easy.
But if both cars are going 40mph then the collision is equivalent to hitting a tree at 80mph. Doubling the velocity does something horrible to the forces involved, I'm sure.
