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The 'Knight Rider' Phenomenon

Oops! My original answer here is wrong! I reserve the right on ocassion to put my foot in it. Anyway, I will keeep my original answer intact, and then print the real one below it.

Date: Thu Nov 28 16:09:04 1996
Posted by: Michael Danekas
Grade level: 10-12
School/Organization: Brentwood High School
City: Brentwood State/Province: Tennessee
Country: USA
Area of science: Physics
Message ID: 849218944.Ph


Message:

As you may have seen on the old TV show "Knight Rider," Kit was able to travel at sixty miles per hour down the highway and drive up into the back of a semi. I do not see how this is possible. If his wheels are rotating fast enough to cause the car to be going sixty miles per hour, even though the semi may only be going one mph slower, as soon as the car hits the ramp, the ramp becomes the road and is moving at zero mph relative to the car. Therefore the car would still be going 60 mph on the ramp and flying out the front of the trailer. Without being able to brake in 40 feet and come to a complete stop, he would get in a wreck every time. If you could possibly explain this to me, I would greatly appreciate it. My brother, a grad student, and my physics teacher say that it is possible, but I cannot agree. Please help clear up this issue. Thanks.


NOTE (June 20, 1997):
the answer I posted below is wrong. I blew it. I have received about 20 emails over the past few months saying why I was wrong, but I haven't had a chance to rewrite this (although I did tell the original questioner I was wrong). PLEASE DON'T EMAIL ME!!! My mailbox is starting to wheeze ;-). I'll update this page when I get the chance.

I have to agree with you on this one, with one reservation I'll get back to in a minute. By the way, I think it's spelled 'Kitt', though I'm not sure. I didn't watch the show much when it was on. ;-)

Say the wheels are spinning such that the car is moving 60 mph. Think of it from the point of view of the car. The road is moving under the car at that speed. Now let the truck ahead of the car slow down a bit so that the car is approaching the ramp (or, from the car's POV, the ramp slowly approaches the car). The wheels of the car touch the ramp. What happens? Well, the wheels are still spinning at a high rate, so the car screeches and zooms up the ramp! Wham! Kitt now needs a repair kit, haha.

Think of it this way: if Kitt were a hockey puck, shooting across the road at 60 mph, then the ramp is moving very slowly relative to it, so slowing down the hockey puck would be easy. But Kitt is not a hockey puck, moving by momentum. Kitt is moved due to the rotation of its wheels. Relative to the road, the piece of tire in contact with the ground is not moving across the road! If it did move relative to the ground, the car would skid. However, that same piece of tire will be moving pretty quickly relative to a ramp moving at the same speed as the car. So Kitt crashes, or at least leaves tremendous skid marks up the ramp.

It would have been better for the ramp to be replaced by a treadmill. The tread could start off spinning at a velocity of 60 mph to match Kitt's ground speed. When Kitt touches the ramp, it immediately starts slowing down. If Kitt brakes correctly, the car can slow down much faster than it could on a static ramp. Better make sure the treadmill is spinning in the correct direction, or else Kitt will accelerate into the truck! Ouch!

My reservation is this: I agree with you that it isn't possible. So how did they film it? I would guess that they did the trick at 20 mph, so the car needed much less space to stop, and then sped up the film. Does this sound right? I think the show is in reruns on cable someplace; I'll check it out and see when I get the chance!


Oops! The real answer:

I blew it. The car won't screech and suddenly accelerate. From the ramp's point of view, the car is moving very slowly. The wheels are indeed spinning quickly, but it would be an easy enough for the driver to hit the clutch (or shift into neutral) before the wheels could appreciably accelerate the car. Kitt would not go careening into the truck at 60 m.p.h. and the driver would not be horribly killed. I have even been told that the car in the show was a rear wheel drive, so the driver would have even more time to disengage the clutch; he could simply let the momentum of the car carry it up the ramp. The treadmill idea was not such a good one; it would actually help accelerate Kitt, which is not wanted. Oh well. So the trick was a simple one, though deceiving, and it got both of us!



©2008 Phil Plait. All Rights Reserved.

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