To clarify, doing anything with excessive force is not the crime. A challenge which doesn't offend against the below with excessive force is fine. (Well except for endangering the safety of the opponent, but that's a rarity)
There are 10 offences which result in a Direct FK.
- kicks or attempts to kick an opponent
- trips or attempts to trip an opponent
- jumps at an opponent
- charges an opponent
- strikes or attempts to strike an opponent
- pushes an opponent
- tackles an opponent
- holds an opponent
- spits at an opponent
- handles the ball deliberately (except for the goalkeeper within his own penalty area)
Carelessly results in a FK only, recklessly a yellow, with excessive force leads to a red.
As an aside, I endorse everything Chris said. Believe me, noone is angerier when I make mistakes than me.