1) 1.35 kg * 9.81 m/s^2 * 18.5 m = 245 Joules of gravitational potential energy converted into kinetic energy. Of course, some of that will be lost to drag, but not (I suspect) very much. To talk of force is slightly inaccurate, because we don't know the length of time over which the impact took place, but the impulse (the total energy transferred) will be about 245 J. (The impact speed, assuming no drag, is 19.05 m/s, or about 40mph.)
2) That may well depend somewhat on things like the angle at which the head is being held (which will control how much elasticity is available in the neck). Since the typical weight of a human head is around 5kg, the change in energy (of your head) is roughly equivalent to emergency stopping from 20mph[1] - but that isn't an ideal comparison because it's a different direction (and the neck's responses in different directions are not the same, for obvious reasons), and because the rest of your body is accelerated when you emergency stop. I suspect the answer is that serious injury is unlikely, but IANAE.
[1] four times the mass, so half the velocity to retain equal KE
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Date: 2009-02-01 04:58 pm (UTC)2) That may well depend somewhat on things like the angle at which the head is being held (which will control how much elasticity is available in the neck). Since the typical weight of a human head is around 5kg, the change in energy (of your head) is roughly equivalent to emergency stopping from 20mph[1] - but that isn't an ideal comparison because it's a different direction (and the neck's responses in different directions are not the same, for obvious reasons), and because the rest of your body is accelerated when you emergency stop. I suspect the answer is that serious injury is unlikely, but IANAE.
[1] four times the mass, so half the velocity to retain equal KE