Originally Posted by
Brian Ratliff
joejack is correct. a = F/m, meaning that there must be an external force on the cyclist to accelerate the mass of the cyclist into the ground faster than gravity. If the front wheel of the bike is
stuck in both the horizontal and vertical directions, then the cyclist can be accelerated into the ground, but only if the cyclist had a death grip on the bars and rode them into the ground - in reality, it doesn't really work this way. Usually, if the wheel is stopped by a curb or stick, the CG of the cyclist just keeps right on moving the direction it was going; bike in tow - the bike might induce a body rotation, but not much as it is only ~10% of the mass of the body. And if the bike gets stuck, the cyclist usually lets go or the bike is ripped out of his/her hands.
Bruce, who is in my club but who I did not know very well, was on a ride when a stick got tangled in his front wheel, causing him to be thrown. It is likely he did not hit the ground any faster than a freefall, but he landed heavily and broke his neck. May he rest in peace. You can read his story
here.
I'm sorry for Bruce, but that's Idiot Physics. If you have a forwards velocity and a downwards one then the impact vector FROM COMBINED MOMENTUM will be the resultant of both, along the hypotenuse of the triangle they make. That gravity acts downwards is irrelevant, although it's nice that you've heard the news.
To give an example, if you were moving forwards at exactly the speed gravity accelerated you to downwards at the moment of impact, you'd hit the road at 1.4 times your forwards speed and at angle - in terms of forces - of 45 degrees. This is NOT a trivial difference for either your helmet or your head. (In particular the helmet shell will get a whack with a component parallel to the adhesive bond between the inner and outer.) And as forwards speed gets higher this difference will get more pronounced.
Yes, I know Brian claims to be a mechanical engineer. But if anyone doubts on me this and can't do the maths for themselves they can go ask in the "Help With Your Homework" section on Physics Forum or some such place.