How hard can a baseball be thrown? Not more than 100mph reguarly.
Salon features some great sports articles from time to time. Recently they published an article entitled, "Pitcher Perfect: Why can't anyone throw a baseball faster than 100mph?"
The piece talks about the physics of baseball and more. Digging in you can find a wide array of pitching articles and some of them are surprising.
The article's author, Noam Scheiber, runs us through the myths of the fastest pitchers and why humans have hit a brick wall in terms of pitching velocity.
The first place I looked was The Physics of Pitching. This is an excellent series of articles talking about what's behind the motions of pitching and how a baseball reacts the way it does. The great thing about this article is that it talks about the rotation of the various pitches.
This site breaks down the formulas for determining the physics of pitching. Excellent resource that does a great job explaining thing.
The final site I found was one that discusses common pitching injuries and their causes. It's a great look into the stresses that pitching in baseball causes on the human body.