Microwave Oven on 5/10/2007 at 14:12
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. The CSI guys were in the lab simulating the event, when one of them was explaining how the event took place. He said that the guys head and the tire tread impacted at a combined speed of 120 miles per hour. When I heard this, I laughed out loud and commented that the maximum possible speed the tire tread could have impacted the guy was 60 mph. My friend said the CSI guy was right and the impact was at 120 mph. We then got into a heated argument about the physics of tire shredding at highway speeds and the relative velocities involved. The argument went unresolved as neither of us would back down on our assertions.
Now, the problem is, I cannot see how the tire tread could pick up a rearward velocity of 60 mph, which combined with the gokart's forward velocity of 60 mph would equal 120 mph. My assertion was that the tire chunk would have a velocity of 0 mph relative to the road, and the cart collided with it at 60 mph (relative to the road) resulting in a combined velocity of 60 mph. So, is my math totally off here? Does a piece of tire have any rearward velocity when it is detached from the tire itself, or is it that any piece of tire coming off would have a velocity that is a combination of the vehicle's forward motion and the rotational motion of the wheel, which would be 0 mph at the point of contact with the road?
I say my math is right. My friend say I don't know what I'm talking about. Who's right?
Gingerbread Man on 5/10/2007 at 14:23
Dude, it's CSI. They can enhance a photo, spin the camera angle, enhance enhance enhance until Deckard wilts in envy, and VOILA! A fingerprint smudge! And it takes fourteen seconds to find a perfect match.
But yeah. Other than the fact that the truck tire would be spinning and shredding, the probable best-case scenario is that it lost ALL forward momentum (which it wouldn't have) and the go-kart clipped it at 60.
But the tire would still have forward momentum and go-karts don't go 60. So the speed at impact (and goddammit "impact" is not a verb) would probably have been way less than 60, never mind 120.
This is what happens when your show's raison d'être is "Who songs, filtered lenses, and trying to have more spin-offs than Law & Order."
(edit: o wait only a chunk of tire)
Well, it's possible that a chunk could be flung backwards... but in reality, truck-shred tends to peel off and kinda floppafloppafloppa rather unhappily and weirdly rather than getting shot backwards.
Scots Taffer on 5/10/2007 at 14:34
CSI CREATED PHYSISC
ignatios on 5/10/2007 at 14:40
The coolest thing about being a CSI is that you can dress however you want, step all over a crime scene and mess shit up, and THEN work in a lab to a kickin soundtrack
n ts n ts n ts n ts n ts n ts n ts n ts
Microwave Oven on 5/10/2007 at 14:43
Well, *I* know CSI physics is dodgy, and their comment about the speed of the tire chunk was lolable, but my friend was adamant that they were right. I mean, really, really adamant. To the point of shouting about it. Wow.
BEAR on 5/10/2007 at 14:57
Mythbusters did a pretty good experiment about truck tires tearing up and decapitating people.
I think they had to send it at the dummy at about 50+mph simulating a 50mph blowout.
My fairly limited understanding would be that its maximum speed would be the tires speed as it blew out, but never double.
Also why were you even watching CSI, its criminally stupid.
Chimpy Chompy on 5/10/2007 at 15:06
Ok, so we have a truck going at 60mph relative to a stationary observer, followed by a car doing the same speed. If the tyre just disintegrated, a piece right at the bottom (so going backwards relative to the truck), would (I think) go flying backwards at a velocity of 60mph relative to the truck and car.
To go faster than that it would need something else adding additional force - like the tyre exploding instead of just falling apart.
TBE on 5/10/2007 at 15:36
Quote Posted by Gingerbread Man
....go-karts don't go 60.....
(
http://en.wikipedia.org/wiki/Kart_racing) I beg to differ ;)
The tire would still be going forward like the truck's momentum, 60 mph. It would have been slowing, so say about 40 mph. The gokart is going forward at 60 mph. I'd say the best case scenario is the kart hit the tire at 20 mph in my physics universe.
Pyrian on 5/10/2007 at 16:41
If the shrapnel bit came off as the tire lost pressure - i.e., at the moment of blowout - the upper limit of its velocity could be much higher than 120mph (since the front and rear cars are going about the same speed, they cancel out, and the bit needs to be going 120mph relative to both of them). That would be highly improbable - tires tend to rip before they shred - but not strictly speaking impossible.
