Thursday, November 10, 2016

Why you should not Build your Own Drag Car

Stevie "Fast" Jackson is a professional stock car driver. His suped-up Ford Mustang, nicknamed Shadow, was known for being able to finish an eighth of a mile track in the four second range and can  reach a top speed of 190 miles per hour. Unfortunately at a recent event in the South Georgia Motorsports park ol' Shadow suffered a horrific crash, while Jackson was able to walk out on the car with only minor injures his car's tires lost contact with the road and flew over 300ft supposedly (I think the reporter might be exaggerating a little here).


The crash can be watched on video at: https://www.youtube.com/watch?v=eBgd_oUswSM
I thought to use what we've learned so far in physics to help me better understand what happened to this car. I began by drawing a basic force diagram for the mustang:


However while I thought that the reason for take off had been due to the high speeds causing a loss of contact and force of friction between the wheels and pavement I quickly realized this could not be the case. There had to be another force in the vertical direction that I was missing that did vertical work on the car. After a rigorous half an hour internet research hunt I discovered that the trickiest part of building your own stock car is the aerodynamics. This mustang needed a bigger spoiler. What occurred to Jackson's Shadow is a rather common, as the car reaches higher and higher speeds the air flowing over the top of the car has a faster velocity than the air below the car causing a difference in pressure. To result of this difference in pressure is that a vertical upward force will be act upon the car explaining why the car took off. This seems to be a concept of fluid dynamics that we have not covered yet so maybe I should of waited later in the semester to post this.

Once realizing the presence of this new vertical force I thought to set out and find the value of this force. I searched online and found that a ford mustang weighs 1735 kilograms. Assuming that the car was traveling at its top speed of 190 mph gives us a velocity of 87 m/s. So the initial kinetic energy of the car was 1/2*(1735 kg)*87^2m/s = 6,566,108 J. I then realized I had no idea how to actually solve this problem so instead I decided to calculate what height the car would reach if all this kinetic energy would have been transferred to potential energy. Setting mgh = 6,566,108 J gives us a height of 386 meters and I stopped here.

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