See Slapstick. Finding humor in someone elses pain, mastered by The Three Stooges it primarily involves people experiencing massive levels of suffering caused by folly or sheer stupidity.

Mel Brooks said it best when he stated, "When I stub my toe, that's a tragedy. When YOU fall off a cliff and die, that, that's comedy!".

There are three steps your brain goes through before laughing at physical comedy:

Someone is hurt.

It checks, and says "It is not me."

And the last step, someone in your brain pushes the button marked "rejoice"