and also divided
by the area (a) of the wounding surface.
In symbols, this is expressed as:
Wounding energy = mv2 / ta
Hence wounding energy decreases if we can increase time and area.
Time. If I take a bullet and touch it slowly to your skin, you
aren't wounded because the marked reduction in velocity means a marked
increase in t. During falls, you slow down the contact by
rolling into it or slapping the mat with your arm like a spring. Even
fractions of seconds of increase in t significantly reduce
Increasing Area. Suppose I take the same bullet
and hammer it into a huge flat sheet of lead and fire it at you (in a
vacuum where there's no air friction) with the same velocity
(v) as a bullet leaving the muzzle of a gun.
The bullet will wrap itself as a flattened surface over your skin (with an area of say 720 square inches). You are extremely unlikely to be wounded simply because of the increase in area (a). Wounding energy is extremely low.
In contrast, if the area through which the kinetic energy is being transferred is reduced to the size of your chin, say 2 square inches, wounding energy is high.
To recap, the larger the mass and the higher the velocity of the oncoming object, the more dangerous the situation.
The larger the area and time of contact, the safer the situation.
We usually can't change our mass or velocity, but we can increase time and the surface area of contact with the oncoming mass, mat, sidewalk, or planet.
The means of doing so are the skills of ukemi.