Two players are going for the ball, and, within playing distance of the ball, one lowers shoulder and drives into other player. This is indeed a foul, but can you point to text in Fifa’s LOTG that would clarify this? It is a charge, of course, so is the only ground on which to blow the whistle that it was careless? I can’t find anything in Fifa’s LOTG about lowering the shoulder being illegal.

Answer (June 21, 2015):
P. 117 of the Laws of the Game:

Careless, reckless, using excessive force
“ Careless” means that the player has shown a lack of attention or consideration when making a challenge or that he acted without precaution.
• No further disciplinary sanction is needed if a foul is judged to be careless

“Reckless” means that the player has acted with complete disregard to the danger to, or consequences for, his opponent.
• A player who plays in a reckless manner must be cautioned

“Using excessive force” means that the player has far exceeded the necessary use of force and is in danger of injuring his opponent.
• A player who uses excessive force must be sent off

Charging an opponent
The act of charging is a challenge for space using physical contact within playing distance of the ball without using arms or elbows.
It is an offence to charge an opponent:
• in a careless manner
• in a reckless manner
• using excessive force

Note: Charging can occur only in the area of the shoulder, never in the center of the back; however, when working youth games, the referee must consider any disparity in height and honor the act if a player is making a “best effort” to follow those guidelines.