Last week, I made a really cool rope swing in the giant old maple in my backyard. It's the kind of swing that has an arc about 30 feet wide, can comfortably go 10-12 feet in the air (and higher if you're not afraid to do it), and is just plain fun. Tucker, who'll be 3 this month loves it. He flies a good 4 feet above my head and screeches and giggles the whole time.
Sounds good, no? Well, we have a neighbor that has four girls that range in age from 1-11. The three oldest, 4, 9, and 11, like coming over and playing with Tucker. However, they love playing with his toys even more. They have been over way more often now that the rope swing is up. Today, I caved in and said I would push them in the swing.
So for about two hours, I pushed the three girls and Tucker on the swing. It was a lot of fun for all, but it did eventually start to tire me out. after a good five turns for every kid, I was pushing the 9 year old girl by running and pushing her over my head as I run underneath. She swings away. I start talking to her sister thinking I'm out of the way.
Without warning, I get a completely outstretched flat foot to the jaw. I heard a loud clicking sound, then I think I was technically unconscious for a split second. I do remember seeing white that turned completely black (I think that's where the term "knock your lights out" comes from, duh). I didn't fall, I hardly moved at all really. I tasted that tooth drilling flavor and blood. One of the girls asked me if I was OK. I said no. Then I walked to the house. I checked my teeth with fingers and tongue on the way there. Lucky for me, I still have all my teeth. The blood is from a hole in my lip. Good times.
Now it may sound like I'm overreacting, but let me reassure you, a nine year old girl on a swing that big could knock out anybody. Let's do the math. A 60 pound nine year old going approximately 25 miles per hour focusing her kinetic energy into a 2.5 inch area equals, I don't know, probably 300 lbs of pressure per square inch. Seriously. A LOT of pressure. I'm lucky I can still eat, but that hurts like the dickens. It's 10 hours later and it still hurts. I have to keep opening and closing my jaw or it sort of seizes shut for a while. I ain't going to the doctor, so don't bother. Worst case scenario, it's cracked and I have to get it wired. I won't give my wife the satisfaction of that.
I found out something important today: I can take a fairly serious blow to the head and still keep my wits somewhat about me. I feel like a real man and all it took was a nine year old girl on a swing.
Tuesday, August 5, 2008
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1 comment:
Glenn,
Let's have some fund with math and physics and do the calculation! For a 60 lb child moving at 10 mph (I'm taking a more conservative estimate), the circular motion formula for net force, from Sir Isaac Newton, is:
Fnet = m x a
Net force = mass (kg) x acceleration (m/s/s of meters per second squared)
Sixty pounds is approximately 27.3 kilograms. To find acceleration, we use the formula:
a = v^2 / R
acceleration = velocity squared (meters/second) divided by the radius (meters).
If we take the radius as the length of the rope, we get 30 feet. 30 feet is approximately 9 meters. Ten miles per hour is about 4.74 meters per second.
a = (4.74)^2 / 9
a = 2.496 m/second squared
We now plug in the mass of the child (27.3 kg) and the acceleration (2.496 m/second squared):
Fnet = 27.3 x 4.74 = 129.402 Newtons.
I'll make another estimation that the surface area of impact between your jaw and the child's foot was about 3 square centimeters. So, the force delivered (in metric units) was 43.134 N/cm^2.
So, one newton per square centimeter = 1.45037738 pounds per square inch... which means that 43.134 N/cm^2 = 62.5602 pounds/in^2.
In research listed at http://www.springerlink.com/content/eh1p3g3ac9m00n2a/, it took between 600 and 800N/cm^2 of force on the human jaw to cause a fracture.
Over all, I'd say you were hit with enough force to give you scrambled eggs upstairs, but fortunately not enough to break your jaw.
Hooray jaw bone density!
Hooray mathematics!
Hooray physics!
Hooray Isaac Newton!
Hooray hard cranium!
And most of all, hooray for the neighbor kid weighing only 60 pounds! If John Goodman had collided with you, this blog wouldn't be nearly as funny!
Now, isn't math and science fun! And to think, I almost gave all this up for the fortune, fame, debauchery, and excess of stardom!
I think I need a beer... or ten...
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