[ck2]

Wind resistance has to be a factor after 25mph, I mean it is on my electric bicycle.

And holy cow, even 25mph is crazy fast, 35? If you tripped and fell at 35-40mph you could seriously injure yourself.

I think I have a good question for the xkcd q&a site: how fast could Usain Bolt run on the moon (ignoring the oxygen problem).

Ah I see someone beat me to the moon problem. Here's another thought, without strong gravity to help return mass to the ground, running is probably much harder.

[maxerickson]

This analysis says Usain Bolt expends >90% of his energy on drag:

http://iopscience.iop.org/0143-0807/34/5/1227/article

(That is, yes, you are correct.)

Answering my own question, he took 41 steps when he set the world record.

[thaumasiotes]

> If you tripped and fell at 35-40mph you could seriously injure yourself.

To be fair, this is quite true at walking speeds as well. Falling is bad. I passed out from dehydration once while standing still; I ruptured my spleen in the fall and almost needed surgery to have it removed (thankfully, it healed while I was under observation).