Originally Posted by
chasm54
Gravity is not a speed. And if you really believe that piling into the ground at 40mph is likely to be no more damaging than doing so at, say, 10 mph, you are making even less sense than before.
It really depends on how you fall. If falling sideways it doesn't matter what your forward speed is, except for torsional and road rash considerations.
Consider a bouncing ball. Drop the ball and it rebounds to a certain height. Throw it sideways and when it falls to the ground it'll rebound to the same height as the dropped ball. The forward speed is unaffected by the impact, except that some spin will be imparted.
However, if you go over the bars, hit a wall or car, or are hit from behind, then forward speed comes into play since the impact will be changing that.
In all cases what matters is the change in speed and direction, not the speed itself.