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Re: [Phys-l] a car physics problem



Hi,
Interesting.
I have heard a claim that a top fuel dragster would win a 1/4 mile race, against a corvette, even if the corvette had a running start. That is the corvette was at its top speed of top speed of 200 mph when it crossed the start line.

Thanks
Roger Haar



On 2/24/2011 10:26 AM, Bill Nettles wrote:
Here's an interesting set of "real world" data which I gave to my students. This is time slip data from a nitromethane fueled funny car. I believe the driver was Ashley Force, and run took place in 2008 before they shortened the nitro cars to 1000 ft.

Data is (position in ft, time in s)

(0,0)
(60, 0.914)
(330, 2.336)
(660, 3.321)
(1000, 4.141)
(1320, 4.858)

I ask the students to approximate each distance interval as a constant acceleration interval and calculate the acceleration during each interval and get a final velocity for each interval. I then get them to convert the accelerations to "gees" and speeds to miles per hour. I soon hear some gasps (0-90 mph in under 1 second), and later a few "what?!"s when they encounter the 660-1000 ft interval (the acceleration calculated from these data is negative). I then let them ponder why that may be the case. I have a guess, but 1) I'm not certain, and 2) I know it's not the whole story.

My guess: I've analyzed several time slips from funny cars this way (constant accel during the interval), and the 660-1000 ft acceleration is always small, but the 1000-1320 is large (again), around 3-4 gees. I suspect that during the 660-1000 ft is the region where air resistance is matching thrust, but the engine (there's only 2 gears, forward and reverse) continues to increase torque and hp so the rate of change of thrust becomes greater than the rate of change of air resistance. Clutch engagement would be another factor. The clutch is programmed to engage gradually, but is fully engaged by the 660 mark. Most of the tire deformation and shake happens in the first 2-3 seconds. (If you've never seen tires on a funny car or a top fuel machine in slo-mo when they launch, you're missing a treat. That's a physics video all by itself.)

If you model air resistance as -k|v|^n, will n change as a function of |v| when dealing with speeds going from 0 to 300+ mph? Does it get bigger or smaller?
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