The forward pass - a study in science

Much has been spoken about the forward pass of late, and we promised to discuss aspects of it.

To kick off we asked a physicist who knows about rugby to talk about it, which he has done with diagrams. So here we have some intelligent explanation, ending with a suggestion for rewording the Law.

THE FORWARD PASS

This happens if the ball, when passed from player to player, travels towards the opponents' dead-ball line.

So for a pass to be legal, it can, at worst, travel directly only across the field of play, i.e. from touchline to touchline, or preferably slightly backwards across the ground.

Consider a ball traveling across the ground as show below, at say 5 m/s. (5 metres per second)

This means it travels across the field of play (from touchline to touchline) at a speed of 4 m/s and backwards (away from the opponents' dead-ball line) at 3 m/s. Put another way, a spectator standing on the touch-line would see the ball traveling backwards at 3 m/s, whilst a spectator standing behind the uprights would see the ball traveling across the field at 4 m/s as shown in Fig 1. Here the speeds of 3 and 4 m/s are known as the components of the speed of the ball.

Consider also a player, P, running with the ball. Then the instant he passes the ball, it has the same speed as the player passing the ball. Assume he passes the ball directly to his right at 6 m/s whilst running at 5 m/s, Fig 2.

The ball actually travels across the ground at 7.8 m/s, as shown in Fig 3, but more importantly at 5 m/s forwards!

Look at players A and B below, with B 1m behind and 6m across the field from A. They are facing the opponents' dead-ball line, as shown in Fig 4.

If they are both standing still, and A passes the ball to B in 1 sec, then its speed backwards is 1 m/s and 6 m/s across the field of play. The balls actual speed, as shown by the arrow, is a little more than 6 m/s (6.08 m/s to be exact).

Now assume that both players are walking forward at 1 m/s, and A again passes the ball to B in 1 sec. as shown in Fig 5.

As the ball leaves As hands, it has a speed of 1 m/s backwards, as before, but because A is moving forward at 1 m/s the ball also has this speed. This means the ball has zero speed in the forward direction and it actually travels directly across the field of play and the pass is legal. But players A and B and the referee (a good one who keeps up with play!) are all moving at 1 m/s, so to the ref. the ball passes backwards as before, but the spectator sees the ball moving directly across the field of play.

Let's now assume that the players and the referee are moving forward at 3 m/s, Fig 6, and the ball is again passed from A to B in 1 sec.

As the ball leaves As hands it has a forward speed of 2 m/s (3 1) m/s. The referee sees the ball passing backwards from A to B, as before, because he is moving with the players. But the spectator cries foul: he sees the ball moving forwards across the field of play at 2 m/s! For the spectator to see a legal pass, the ball must at least have a backward component of 3 m/s (the ball would then travel directly across the field of play). This means it covers the back distance of 1 m between A and B in 1/3 sec. In turn this means the ball reaches B, covers the 6 m across the field in 1/3 sec and so the speed across the field is now 18 m/s (6 divided by 1/3) and the actual speed is a little over 18 m/s (18.25) or about 65 km/h! Passing a ball at this speed whilst running at 3m/s (11 km/h) is not easy! And 11 km/h is not fast.

Normally a wing, or other back, running at a fair speed, say 5 m/s (18 km/h or running 100m dash in 20 secs) would need to pass the ball to B at 165 km/h for the pass to be legal. The ball would need a backward component of 6 m/s, and so the back time would be 1/6th sec and the speed across would be 36 m/s. Of course, if B were to move further back, then the passing speed would decrease, but then it would have to be increased if B moved away (across) from A. A really fast back, probably running at say 8 9 m/s (or a 100 m dash in 11 12 sec), would have to pass the ball to his team-mate at nearly 220 km/h for the pass to be legal.

These things become particularly clear to spectators when the ball is passed between running players as they cross the 10 m, half-way or 22 m line. It is fair to say that virtually all passes between running players will be forward since the balls travels towards the opponents' dead-ball line during the pass. This is what is seen by stationary observers, spectators, but not be the referee who is running with the players.

I would like to suggest the law read:

The player passing the ball, passer, must be in front of the player receiving the ball, receiver, at the moment that the ball leaves the passers hands. Simply put passer to be in front of receiver.

Another interesting, related, observation.

Often when a player is about to be tackled he will try and pass the ball. In the tackle the passer is likely to be slowed or brought to rest (if tackled hard). The referee. also slows or stops and the ball then has to carry on at its original speed (since it has just left the player's hands) and the player is deemed to have lost the ball forward: scrum/advantage! Answer: when about to be tackled, hold onto the ball! Better still, pass it before you are about to be tackled!

By Case Rijsdijk