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-   -   Maximum Speed of a Porsche (http://www.physicsforums.com/showthread.php?t=302653)

acws337 Mar26-09 01:04 AM

Maximum Speed of a Porsche
 
1. The problem statement, all variables and given/known data

A Porsche 944 Turbo has a rated engine power of 217 hp. 30% of the power is lost in the drive train, and 70% reaches the wheels. The total mass of the car and driver is 1530 kg, and two-thirds of the weight is over the drive wheels. What is the speed of the Porsche at maximum power output?

2. Relevant equations
P=FV


3. The attempt at a solution
I calculated the maximum power by taking 70% of 217 and multiplying by 746W/hp = 113317.4W. I don't know where to go from here, since I'm not sure how to obtain a value for F, the forces doing work on the Porsche at maximum velocity. It's not simply rolling friction, since plugging in (2/3)(1530)(9.8)(.02) (.02 is the value of rolling friction listed in the text) for F yields a maximum speed of 566.8 m/s.... I would think that there would also be a drag force doing work at maximum velocity, but the dimensions of a cross section of the car are not given. Any ideas on how to go about finding vmax?

acws337 Mar26-09 01:23 AM

Re: Maximum Speed of a Porsche
 
Well, I found the correct answer....but it doesn't make any sense to me. Using the friction force that accelerates the car as F (2/3)(1530)(9.8)=9996N, I found v to be 11.34m/s. That seems awfully slow for a sports car and for a car at maximum velocity I didn't think that static friction would be doing work.


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