Earlier this week, my friend Russell was run over by a woman driving an SUV. He was cycling down a road in Washington, D.C. She was a doctor, trying to leave Georgetown Hospital. She didn’t see him. Russell was knocked from his bicycle, and she drove over his left leg.
Before we go any further, I should tell you: Russell’s fine. One-hundred percent intact. That’s the first problem with running him over. I cracked my humerous and underwent surgery after slowly running my bike into a curb. Russell was struck by a car, and then run over, and it didn’t leave a mark. That’s not exactly fair.
The second problem is that Russell now thinks he’s invincible, and that’s a bad attitude for a cyclist to have. Fortunately, there’s some physics we can draw on to explain what really happened that night.
The car ran over Russell’s femur, the strongest bone in the body. The internet suggests a femur can withstand up to 600 lbs of vertical pressure. Russell is a young man who doesn’t suffer any particular bone abnormalities, and we know he rides his bike. So we can assume that his femur is of at least average strength.
A small SUV might weigh an imperial ton, or 2000 lbs. Divide that into the weight supported by each tire (500 lbs), and you find that a small SUV would have trouble breaking Russell’s leg.
However, a large SUV could weigh as much as four tons, or 8000 lbs. It could do some serious damage. That’s where the suspension comes in. If only one tire rolls over the leg, the wheel will simply compress the suspension on that side, and distribute the extra weight of the car to the other tires.
Just how far can the suspension be compressed? A PT Cruiser has an 8.5″ suspension traveling distance. That’s certainly higher than the height of Russell’s squashed leg. The suspension on a Land Rover can travel a respectable 7″. In short, without even considering the supporting role of the ground under Russell’s leg, we’ve determined his limb was already likely to survive the accident intact.
Unfortunately, just like for every other great feat ever performed, a simple physics explanation lies just behind the magic. I’m just glad the physics worked, and I didn’t have to write a eulogy instead.
Hey, that femur’s WELL above average!
This is awesome. Thanks for breaking it down (and taking some wind out of my “nothing in the world can hurt me” sails).
also you left out the fact that my sick-ass meaty quadricept absorbed a lot of the impact and cushioned my precious femur.
this is awesome. i wish everything in the world were narrated in this way!