A tripping penalty in football is called if a player uses their lower body to obstruct a player on the opposing team. Tripping penalties can be called on both the offense and the defense.
For committing a tripping penalty a team will be assessed a loss of ten yards. If the tripping penalty is committed by the defense an automatic first down will be given.
SECTION 40 TRIPPING
Tripping is the use of the leg or foot to obstruct any opponent (including a runner)NFL Rulebook 2021
As you can see from this definition tripping penalties can be called for tripping those with and without the ball.
Tripping as an illegal block
Oftentimes an offensive play is going to make or break by the success of a few key blocks.
When a blocker knows they are not going to effectively block their player they may become desperate. One way to salvage a block is to stick out your foot or leg in order to trip the defender before he can reach the ball carrier.
This is not an incredibly effective strategy but every once and a while an offensive player will be able to eliminate a defensive player from the play via a trip.
Unfortunately, you are not allowed to block defensive players by tripping them. These fouls are usually quite obvious as it is usually defenders who are about to make a play that are fouled.
If the ball carrier is not yet past the line of scrimmage when this foul occurs then the penalty yardage will be assessed from the line of scrimmage.
If the ball carrier is past the line of scrimmage then the penalty yardage will be applied based on where the ball was when the penalty occurred.
Tripping penalty on the defense
Another way that a tripping penalty can be called in football is against the defensive team. This penalty is called when a defensive player sticks out his leg in order to trip the ball carrier.
Generally speaking, this penalty is committed by a defensive lineman when the running back is making his way through a hole.
If the defensive lineman is tied up he may find himself close to the ball carrier yet unable to make a tackle. In these situations, linemen are tempted to stick their leg out in order to trip the defender up.
This is of course illegal and is almost always called by the officials.
When this penalty is committed by the defense the offense gets to move the ball forward ten yards and is given an automatic first down.
Why is tripping illegal in football?
There are two main reasons that tripping is illegal in football players safety and fairness.
In terms of player safety tripping opponents is simply not the safest way to bring them to the ground.
If defenders stuck out their leg each time a ball carrier went by it would be unsafe for both parties.
Defensive players were surely get knee and foot injuries as their legs would receive constant contact throughout the game. And ball carriers would be subject to quick trips that would send them violently to the ground.
The game of football is physical enough already having tripping as a legal action would only lead to more injuries.
Fairness of play is also another major reason why you are not allowed to trip opponents in football.
In general, tripping is used as a last-ditch effort when you have failed to complete your responsibility on the play.
Imagine if each time a running back ran through the defensive line you saw four defenders sticking out limbs to trip his feet. This is simply not the style of football you want to watch on television.
To bring down another player in football you must tackle them or push them out of bounds. Stick out a limb into their path to trip them up is simply not how football is played.