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Old 10-15-2005, 07:14 AM   #1
Alexander Karatis
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We placed a bet with a friend of mine, inspired by David Hasselhoff's signature move in Knight Rider where he would enter the back of a moving truck with KIT while himself moving.

My friend claims that if the truck is doing 80km/h and KIT is doing 85 km/h, then the moment KIT hits the truck's ramp, it is doing 85km/h relative to the truck, and 165 km/h relative to the road.

I claim that it will be doing 5 km/h relative to the truck, and 85 km/h relative to the road.

Who is correct and who is wrong? But most importantly, we need credible sources with theory and example to prove the validity of our argument.

Thanks in advance!

(P.S.: I'm SOOO sure I'm right I can hardly control myself!):biggrin:
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Old 10-15-2005, 07:26 AM   #2
Alexander Karatis
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Interesting LInk-it appears I am right...

He so going to pay for this!!!

http://www.knightrideronline.com/phpbb/viewtopic.php?t=2848&highlight=moving+tru ck
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Old 10-15-2005, 07:57 AM   #3
Jeremy Bloniasz
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Is this the same as saying that if I'm driving 70 miles per hour and throw a baseball out the window at 60 miles per hour, the baseball is now going 130 MPH?

-Jeremy
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Old 10-15-2005, 09:19 AM   #4
James Falkner
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Assuming KITT's wheels can handle instantaneous acceleration from 0 to 85, then yes, he would insantaneously go from 85 to 160. But the front wheels cannot take that kind of acceleration.

reason why it is instantaneous, is that from the truck's frame of reference, KITT is going 0 (or close to it) just before he hits the ramp, but the wheels are spinning at 85. So for the wheels not to peel out, he would have to accelerate from 0 to 85 (thereby keeping the wheels in non-skidding contact with the moving ramp). KITT likely cannot take this (no matter how corny his jokes are) so the wheels have to peel out.

Bottom line: you are both right. There is an extremely short period of time where KITT would be going 165, but it is on the order of nano seconds before the wheels immediately start skidding and the car goes from 170 to 85 very quickly (or a little over 85, enough so that the car does go up the ramp). So you guys have to split the pot on that bet if you want to be absolutely positively dorkey-listically accurate on "who's right".

Reminds me of the time I spent over an hour trying to convince my friends that a bag of unopened (i.e. airtight) potato chips will inflate when you depart in a commercial passenger aircraft *from sea level* (they pressurize, or more accurately DE-pressurize) the cabin to the equivelant of the atmospheric pressure at 8000 ft, cuausing the pressure in the bag to be greater than outside the bag, and therefore inflate. Same reason why airplane doors would blow INWARDS if a malfunction caused the interior pressure to remain at the equivelant of 8000 ft. of altitude (remember that stewardess who claimed she was sucked out in Miami after this happened? It was B.S.)
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Old 10-15-2005, 09:21 AM   #5
Brad Hirakawa
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"Is this the same as saying that if I'm driving 70 miles per hour and throw a baseball out the window at 60 miles per hour, the baseball is now going 130 MPH?"

Well... there is that pesky Lorentz transformation. But if velocity is low enough with respect to the speed of light, then the equation you mentioned (70+60=130mph) should do.

Unless of course you happen to be traveling at the speed of light. Then, if you threw the ball forward at 60 miles per hour, the baseball would still be going the speed of light. :-)

Brad

ps. I took physics during summer school, so this may be very wrong.



(Message edited by hirakawa on October 15, 2005)

(Message edited by hirakawa on October 15, 2005)
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Old 10-15-2005, 09:28 AM   #6
Brad Hirakawa
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James,
Finally something useful to do with a bag of potato chips, besides reading the label as an example of what not to eat! :-)
Brad
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Old 10-15-2005, 11:35 AM   #7
Chris Goodrich
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KIT will be doing slightly more than 85KMH with respect to the road, and slightly faster than 5 KPH with respect to the truck. KIT is carrying two types of inertia, linear (forward inertia) of its total mass, and rotational inertia of the wheel assemblies (and whatever parts of the powertrain are engaged, but we'll ignore that and the engine to keep things simple). Because there is no external force, the linear inertia remains the same so KIT would be going 85KPH with respect to the road. However, the rotational inertia of the wheels makes them still want to spin at the same RPM as on the road, which no longer matches the speed with respect to the truck. They will slow down to the speed relative to the truck releasing the energy stored as rotational inertia in 3 ways 1) spinning the tires against the truck, releasing energy as heat and sound; 2) speeding up KIT slightly (rotational inertia only involves the weight of the wheels, so the force is low relative to the total mass of the car); 3) slowing the truck even slightlier (same reason as #2, but the truck is more massive). Wow, I feel like a nerd.
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Old 10-15-2005, 12:41 PM   #8
Jeremy Bloniasz
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Brad, I never took physics so let me understand. As long as I'm not going the speed of light the 70+60=130 formula works out?

Jeremy
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Old 10-15-2005, 01:06 PM   #9
Mike Yukish
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Bottom line: you are both right. There is an extremely short period of time where KITT would be going 165, but it is on the order of nano seconds before the wheels immediately start skidding and the car goes from 170 to 85 very quickly (or a little over 85, enough so that the car does go up the ramp).

Uhhh, no.
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Old 10-15-2005, 07:20 PM   #10
Brad Hirakawa
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Jeremy,

I'm having bad flash backs to those classes.

As far as we can perceive... I think the answer is yes. Which is why I wrote ..."if velocity is low enough with respect to the speed of light, then the equation you mentioned (70+60=130mph) should do.

I have a headache now. I barely got by with C+ and B-'s it those classes.

Brad
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