ASK MRMARY || (Physics) Can You determine how Fast a Car was going by the Skidmarks it leaves ?


originalNo but I can tell what you ate 10 hours before the driver jammed on his breaks

Seriously Now

The most important thing you can tell from the length of skidmarks (giggle)  is how fast a car was going before it started decelerating. But in order to explain how that works, I have to run some seemingly obtuse words at you.

Kinetic Energy – Think of it like this, things in motion possess a certain energy that is a function of their mass and velocity. A 320 lbs line back running at full speed has more kinetic energy than a 92 lbs women on the Atkins Diet skipping through the dog park with her overly yappy yorkie.

Coefficient of Friction – By this point in your life, if you had had the pleasure-numbing pleasure of using a condom, you my friend know all about what friction is. Depending on the surface the force do to friction will be different. So  the coefficient of static friction across a rough surface will be a lot higher than a teflon coated surface. 

Work – Work is a something many Americans right now do not know about, not only because of the economy but also because we suck at science. Anytime a force causes something to move we can say work is be done. If I apply a force to a box and moved it 100 feet, I have disproved the idea that Blacks don’t work and are lazy.

Force – something which influences a mass in such a way that it moves 

The Metric System – Most of the world use the metric system, but fuck them right cuz this is ‘Murica?

Solving the Problem

Some douche bag, insecure about whether or not he can please the skank he is shacking up with, with his man-meat, buys a fast car and proceeds to drive through a residential neighborhood like a man possessed.

He sees a kid run after his dog across the street and slams the break, and leave some serious skid-marks, like Sunday afternoon dairy binge kinda skid-marks. You wanna know how fast this fucker was going just in case you, your friends and their baseball bats see him again at a red-light. This is how you do it, although if you are the type seeking to teach respect with a baseball bat, science may not be your forte.

  1. Measure the length of the skid-marks in feet and convert that to meters. Let’s say the skid-marks were 15 feet, that is 4.6 meters.
  2. Take these equations, all of you and use them. This is the product of centuries of thinking, use this so that speeding violations may be fought and forgiven

Work = Force * Distance => W=F*d
Force  = Mass * Acceleration => F= μ*m*a ||  μ = that coefficient
Kinetic energy = ½ Mass * (Velocity) Squared aka KE = ½ mv

  1. Set up the equation this way: The work done by Friction (tires rubbing against the road) is equal to te kinetic energy of the moving vehicle

W = KE
F * d = ½ mv2
μma*d = 1/2mv2        || “a” here is the acceleration due t gravity and is = 9.81 m/s2  

use algebra to find an expression for v and solve for it, the weird looking m is the symbol for the coefficient of friction and 0.7 in this case

Brief Note: This is my algebraic expression for v, and my work

v = √(2*μmad/m)
v =√2μad
_______
v =√2 (0.7) (9.81 m/s2)(4.6m)
v = 7.95 m/s or approximately 18 mph

According to the NYS Dept of Transportation he was under speeding limit. So then if you and your friends chose to teach him lessons with your baseball bat, it’s on you.

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