In a collision, you have three or four sub-collisions all taking place in sequence. First, the vehicle hits some object. The vehicle abruptly slows, but unrestrained objects inside it continue at the same speed, in the same direction. Then the unrestrained body hits the interior of the vehicle, and starts to slow. That’s the second collision. That body’s internal organs are still moving at speed until they hit the inside of the chest (or get cheese-sliced by their supporting ligaments–and that’s where you get things like bisected livers or aortas). The fourth collision is when the bowling ball you left on the rear deck hits you in the back of the head, because that continued at the same speed in the same direction. Newtonian physics: Learn it, live it, love it.Link
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.