Knight Rider....

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Knight Rider....

Postby bettina on June 7th, 2007, 10:15 pm 

Knight Rider....

A friend and I were watching the progam "Mythbusters" where they wanted to prove one way or another whether the character Michael Knight could have really driven his car up a ramp and into the open rear of a tractor trailer doing 60 mph on a highway so the car would come to rest inside it. The ramp is connected to the tractor trailer and is at aproximately 30-35 degrees. Thats a guess, but it looked about that. Just picture this with a steeper ramp.. [img:800:600]http://www.rockys.cc/PC160008.JPG[/img]

Anyway, they did it. With both the truck and car doing aprox 60-65 mph, the car drove up the ramp without a problem. The cars tires were striped so you could see it rotating when they ran it in slow motion and I didn't see the tires spin at all.... it looked normal.

What am I missing?..... To me, when the rear tires (rear wheel drive car) hit the ramp, the ramp should appear stationary in relation to the tires and should have lurched the car forward enough to crash into the inside of the trailer and I should have seen the tires screeching on the ramp while going up... My soon to be ex friend tells me I'm wrong.

Bettina
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Postby Lincoln on June 7th, 2007, 10:25 pm 

It depends on the coefficient of friction. The tires could easily spin and slip on the ramp. It depends on how slippery the ramp is.

In fact, if the ramp is reasonably slippery, it would be mostly just inertia doing the job.

I remember the show from when I was younger, but don't recall much about it.
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Postby turbine on June 8th, 2007, 7:15 am 

When you see a car spinning its wheels from a resting stop the engine is in a very high rpm and the transmission is gear for high toque. When a car is traveling at 60 mph the engine is at low rpm and the transmission is geared to low torque. When the wheels hit the ramp the only energy that is absorbed, is the inertial energy in wheels, transmission and engine. This is very low compared to the kinetic energy of the car. If the car is resting it required all of the energy the engine can muster to overcome the friction the tires have on the ground. At 60 the engine energy is low and there is not much stored energy in the drive train to spin the wheels.

Hope this helps.
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Postby bettina on June 8th, 2007, 1:40 pm 

Thanks to both of you.....

I don't know enough about cars and thought the rear wheels were really doing work. I visioned the tires doing 60 with lots of power which would propel it like a rocket. I see now that they are doing very little work which is why in the video I didn't see the tires spinning. It makes sense now.

Thanks... I have to go eat crow now... :)

Bettina
bettina
 


Postby nikomaster on November 1st, 2007, 10:14 pm 

You dont have to be an expert with cars just simple physics
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