✔ 最佳答案
The energy of the deceleration.
The Force acting on a falling body is its weight. On Earth, this will result in an acceleration of 9,8 m/s per second.
Falling from 100 m, it will take you 4,5 seconds.
Therefore, your speed (when you reach one millimetre above the sidewalk) is a tiny bit over 44 m/s (almost 160 km/h, approx. 100 mph).
The constant force needed 4.5 seconds to bring you up to that speed.
However, the sidewalk will stop you in the time it takes to squish your body by (less than) half your thickness; let's say a quarter of a metre.
Your speed will have to go from 44 m/s to 0 m/s over that distance of 0,25 m.
The time it takes for the sidewalk to stop you depends on the upwards acceleration imposed by the sidewalk.
0,25 = (1/2) a t^2
44 = a t
solving both equations for a, we get
a = 2 * 0,25 / t^2
a = 44/t
Since a has the same value for both equations:
0,5 / t^2 = 44/t
cross multiply
t/2 = 44 t^2
t = 88 t^2
since t is not zero (you cannot stop instantaneously - your body does take some time to go squish), we can divide both sides by t
1 = 88 t
t = 1/88 of a second
Then, going back to v = a t, and knowing the values v=44 and t = 1/88, we get
a = v/t = 44 / (1/88) = 3872 m/s^2
The force with which the sidewalk stops your body is almost 400 times your weight.
THAT is the force that stops you.
It is not so much the fall that kills you... it is the sudden stop.