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The physics of getting outkicked



blog-0585835001380686784.jpgAs a competitive runner, there is nothing more annoying than being passed at the very end of a race, and nothing more satisfying than doing the passing. The final surge of speed before the finish line is commonly referred to as a "kick" and runners who can consistently run the last 100 to 200 meters of a race in a convincingly Usain Bolt-like fashion are known as having "a kick". Getting passed by someone while they are "kicking" faster than you is called being "outkicked" Unfortunately I'm not fast when it comes to raw speed and therefore don't have much of a kick. As a result, I have been outkicked on more times than I care to count, leading to much regret and humiliation. My latest encounter with this phenomenon occurred at the McQuaid Cross Country Invitational last weekend. For 2 3/4 miles I had been fighting myself and a pack of aggressive cross country runners (a bit of an oxymoron...) for second place, and I thought I had broken away. Then, coming down the last hill before the final straight away someone catches up and I hear his coach yell "rock that 52 speed!" I groan inside, now knowing that the guy can run 400 meters in 52 seconds which is very fast, much faster than me. And that's where physics comes in.

The maximum speed at which a person can run is determined by newtons 3rd law, which states that for every force applied, there will be an equal and opposite reaction force. While running, the foot and leg muscles apply a force to the ground in the opposite direction of the intended direction of travel, and via newton's 3rd law the ground applies an equal and opposite force to the runner, propelling then forward. The reason that my competitor was faster than me was that he was able to apply more force to the ground and therefore receive a greater push forward from the ground. The reaction force received from the ground because of newton's 3rd law is translated to speed by newton's 2nd law which states that net force equals mass times acceleration, or acceleration equals net force divided by mass. Assuming that me and this other runner are about the same mass (because, being runners, we're both unnaturally underweight) the other runner is able to accelerate and run faster than me because he is able to apply more force and is therefore subjected to greater net force in a forward direction. And so, once again, I got outkicked (but on the bright side I still took 3rd in the race).

This just scratches the surface of the physics of running, so stay tuned for more; but I'll mix it up with some other cooler stuff too.

~That dry, boring running guy


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Great blog post!  And if it makes you feel any better, I got "kicked" dark and early this morning by a gal about half my size...  

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