Originally Posted by Dekz
9.8m/s is the ACCELERATION of gravity. So if he is only falling for 0.15 seconds then he will not reach a speed of 9.8m/s and therefore your calculation of 0.15 seconds is incorrect. Your overall concept is correct, the guy is going to stop MUCH faster than the car, however your math is wrong. Sorry.
yup. i realized my math was totally off :doh::doh::doh::doh::doh:. looks like i need to go back to skool too
i'll figure it out tomorrow. but to get an idea of how long it would take; take a rock, drop it from about 4-5 feet in the air, and see how long it takes to hit the ground. it'll be under a second, for sure.
Originally Posted by agent87
Also, a human body hitting the ground isn't going to just stop. Why? Because of the very same law you mentioned. You're forgetting about the fact that the kid will be carrying the same horizontal momentum that threw him off the car when it hit the brakes. Ever seen a motorcyclist lowside on a racetrack? They go a-slidin'. Thank goodness for leathers. Furthermore, I'm pretty sure the coefficient of drag of flesh and bone isn't nearly as high as that of the rubber in the cars tires. So provided the car continued to brake (afterall, isn't that why the kid flew off the hood/sunroof?), it should theoretically stop more quickly than the kid tumbling on the road unless it hit a patch of ice or oil that the kid didn't. Just some food for thought.
you're right that it isn't going to just stop, but he isn't going to slide that far. for one thing, when motorcyclists lay their bikes down, they're typically going at extreme speeds - especially on the track. the leather also helps to reduce the friction between the asphault and the rider, to minimize the damage from sliding.
the car can theoretically take longer to stop
not because of the coefficient of friction between rubber and the asphault, but because the car carries too much momentum with it. the energy required to stop a car is enormous compared to the energy required to stop a human. just look at the examples i gave earlier.
an average car traveling at 40 mph takes an average of 120 feet to come to a complete stop. so in order to avoid getting hit, he'd have to travel more than 120 feet. take into account the time it'll take for the human to fall to the ground - under one second (lets say exactly one second). initial velocity is 40 mph, which if i'm doing my math correctly this time, is .0111111... miles/second. so in one second, you travel about 58.67 feet. that means he travels 58.67 feet before he hits the ground. then, in order to cover the rest of the 120 feet, he'd have to slide another 61.33 feet (and then some). 61.33 > 58.67. he would have traveled the first 58.67 feet with almost zero friction. the remaining 61.33 feet would have to be covered with considerably MORE friction, being flesh on asphault. again, 61.33 > 58.67 - and this thus is impossible. so he would have stopped before the 120 feet it would take the car to stop, and he could be hit.
again, this isn't meant to be proof that it did or did not happen, just that it
can. certain people think it doesn't make any sense and is physically impossible, where in reality, it is entirely possible.