I don't understand Rugby, Australian rules, or Gaelic football especially well (or US football for that matter, though I've watched it casually over the years), but my understanding is that tacking in these sports has a different purpose and that tackling technique reflects this. You may be on to something with you comment about yards - in American football, it can even be a game of inches. As a result, tackling isn't a way to halt play or stop someone, or to transition during flow of play - you need to essentially completely halt that person's forward movement without even giving an inch.

I've thought this may be why American football has more of the full body collision, where two (very large and powerfully built) men collide at high velocity almost horizontal to the earth. They're trying to gain as little as a few inches in the collision.

I've heard complaints from Australian rules players who come to the US about tackling - that the way US players have learned to tackle doesn't have much strategic value and greatly increases the possibility of injury, buy they continue to tackle this way when they join Australian rules leagues.

That's what I've heard and thought about, I do want to be clear that I don't know too much about all this - what I've written is more of a question in case someone in the know could weigh in.

There is another factor at play here: Rugby, Australian rules, and Gaelic league wear next to no equipment. The problem with American Football is the players are geared up with heavily padded helmets. This has the effect of turning heads into battering rams. You wouldn't see 2 rugby players run into each other full steam because they don't have gear and would break bones and get cut. The gear lessens brain impact but does not eliminate it. So you then get higher impact forces that appear less brutal due to non visible trauma to the brain that is being rattled the whole time.

Another difference on tackling in Rugby: you can only tackle the ball carrier.

Not sure what you're saying here...are you equating blocking and tackling?

In American football you also may only tackle the ball carrier. You can initiate contact other guys (usually in the form of blocking), and that can be very violent at times...but I don't think there's a scenario where you can tackle someone without the ball (and it not be called for a penalty).

In rugby there is no blocking. The only time non-ball carriers come into contact with the opposing team is during a scrum or accidental collision (which are quite rare).

Or in Rugby Union, during a ruck or maul.

The main difference in tackling technique between American Football and both codes of Rugby is use of the arms. In Rugby the arms must be used to tackle the opposing player. A shoulder charge, with no arms used, is illegal and may result in expulsion from the game. In a recent New Zealand vs British and Irish Lions game, NZ player Sonny Bill Williams was given a red card and sent from the pitch for this exact offence.

The other main points to note are that the opposing player can only be tackled below shoulder level and, as you've pointed out, in open play only the ball carrying may be tackled (or blocked).

Tackling in American football is almost gone from the game. The object now seems to be to smash into the ball carrier as hard as humanly possible and hope he falls down. You rarely see a receiver or quarterback tackled, you see the defender simply run into him as fast and hard as he can. Watch some film from the 50's and 60's and you will immediately see the difference.

This is because defensive scoring opportunities (turnovers) are so valuable in American football that defenders are pushed to do whatever they can to get the carrier to fumble, and this usually means launching themselves "at the ball" every time.

Tackling the ball itself and not the player.

Or a ruck - clearing out the ruck, it's allowed there. Or a catch and drive in the lineout. But yes, the contact in these situations is more pushing than collisions.

Yes. Far less physical however. You can 'join' a ruck, but not tackle full steam, let alone a shoulder charge or direct hit.

And even the scrums have reduced contact nowadays; arguably this has resulted in them becoming rather difficult to execute well, and they're becoming a bane to the modern game. Tweaking these things does have knock on effects.

I think there's been a positive change since Rugby Union scum engagements went from "Crouch-Touch-Pause-Engage" to "Crouch-Bind-Set". The initial contact is not as violent, and it would seem far fewer scrums are collapsing, especially on the point of engagement.

I don't have stats to back this up, but my general impression over the last couple of seasons is that less scrums require resetting.

The other big difference is what's obsessively measured.

A few yards makes a huge difference in rugby, and players are certainly highly incentivised to give it their all, but even for the battering ram-type players, their ability to help the team actually get the ball across the gain line is a function of the stamina and mobility to get around the pitch for multiple phases of play, complex situational awareness and some degree of ball handling skills, not just the ability to block off the man directly in front of them for a few plays.

Not to mention rugby's increasingly strictly applied and penalised rules on what is and isn't a legal tackle which mean that a player who doesn't tackle in a relatively safe manner will be a liability to their teammates as well as themselves

What?

There are just five men on the ice for a team in hockey, there's arguably more pressure for each individual player to "give it their all" at any one moment than in football where there's 11 men on the field.

As someone lucky to be from Pittsburgh where we have both the Penguins and Steelers consistently being the top teams in the NHL and NFL respectively, there is no shortage of serious brain trauma in the NHL. Just ask Sidney Crosby.

s/difference of a few yards/difference of 1 yard/

signed, this totally not still upset Seahawks fan.

(re: 2015 Superbowl)