In American football, tripping is a rare penalty. Anyone on the field must trip one of their opponents in order for this to be called. This foul can occur both during and after a play. While tripping is normally done by someone who is not in possession of the ball, the penalty also applies to the runner. The purpose of this rule is to prevent players from using unfair tactics to gain an advantage over their opponents.
There are two types of tripping penalties: unsighted tripping and sighted tripping. On either type of trip, a player cannot use his or her arms or hands to cause a fumble of the ball. However, a player can use his or her body to attempt to stop another player's movement by sitting or lying in his or her path. This is known as a blindside block. Tripping is not considered a foul if it is done as part of a legal play or strategy, such as a tackle formation where one player is assigned to trip the other player who is trying to catch the ball.
Tripping is generally not considered serious misconduct unless it is repeated (three times within a game, four times in total). At that point, it becomes a foul against the offender. Players are usually given a warning before being ejected from the game. Ejectionions usually result from several tripping violations in one game or multiple games in a season.
Kicking, tripping, and shoving are the fouls. On the soccer pitch, physical contact between opponents is permitted and common. A foul may be called if, in the referee's judgment, one of the players is negligent or irresponsible, or if he or she utilizes an arm, hip, or dipped shoulder to unjustly push the opponent off the ball. Pushing with the hand or arm is not a foul; only kicking, punching, and head-butting are prohibited actions during play.
When you push your opponent away from you, you are being responsible and fair. You are not trying to hurt him or her by doing this. Instead, you are simply making room on the field for your team to play. Remember that it is all part of the game!
There are times when you should not push your opponent. For example: If you are about to kick the ball and need space to shoot, you should not try to push your opponent out of the way. Also, if you are about to tackle someone and need space, do not block their path. Only call a foul if there is no chance of your player keeping control of the ball.
Pushing does not always mean a foul will be called. Sometimes you want your player to be pushed so they can have more room to run. This is called "tagging" and it is usually done by a defender who wants to win possession of the ball but does not want to risk a penalty by tackling his or her opponent.
If a thrower throws a forward pass with no realistic possibility of completion while facing an impending loss of yards due to defensive pressure, it is a foul for deliberate grounding. The flag should be thrown by the umpire when he believes that a loss of yardage would result if the quarterback continued to play the ball.
An example would be if a quarterback threw a forward pass into double-coverage with no receiver open and was subsequently tackled in pursuit of the ball. In this case, there is no chance of the pass being completed and so it is a foul for deliberate grounding.
The reason for this rule is to prevent a team that is losing from continuing to play unless necessary for survival. This includes situations where a team is close enough to field position that further play could potentially change the outcome of the game (e.g., down 3 points with 1 second left on the clock).
Deliberate grounding is not called if the quarterback drops back to pass but then immediately throws the ball forward while still facing pressure from the defense. In this case, there is still a chance of the pass being completed and so it is not considered intentional groundings.