Power / weight calculations

[Scarfman]

Bruce, we're mostly talking about billard balls here. Velocity (speed) is all that matters, not whether the ball is speeding up or down. But with your human units, I reckon the force of a person slowing up to 10 m/s will be less that someone speeding up to 10 m/s. The body changes shape in speeding up and slowing down.

 

[Spewn]

I think you may be missing the point about gravity in the equation. When A Fainga'a tackles he dives at speed downwards to the feet. The vector of his dive plus the vector from gravity combines to produce a forceful ankle biting tackle that knocks down even the biggest backs.

 

[Bruce Ross]

Bruce, we're mostly talking about billard balls here.

If so, Scarfman, the model you're using is totally inappropriate and inadequate for making any meaningful statements about the real world of rugby collisions.

 

[DPK]

If so, Scarfman, the model you're using is totally inappropriate and inadequate for making any meaningful statements about the real world of rugby collisions.

Never let the truth ruin a good equation...

 

[Henry]

G force?

 

[Scarfman]

If so, Scarfman, the model you're using is totally inappropriate and inadequate for making any meaningful statements about the real world of rugby collisions.

No shit Sherlock.

 

[Scotty]

So,

If kinetic energy is the correct way to measure the force of the collision, then it means that speed is a lot more important than mass. If it is momentum then they are of equal importance (in the sense that changing them will result in a proportional change in momentum).

 

[Scotty]

Groucho, could you convert that to Newtons?

(Edit) I believe 1 joule is equal to 1 N-m?

By Groucho's calcs 1 joule would be equal to 10 N.m2/s2

 

[Ruggo]

Would there be any chance of knocking up some footage of some of the players in question to provide some perspective between the math and the players movements? Would be interesting to compare with the players technique in contact. Fainga'a, AAC (Adam Ashley-Cooper), Carter and Digby would all make for interesting viewing.

Great thread fellas.

 

[Groucho]

I've had a chance to mull over this with a glass of shiraz and I now reckon both measurements are needed to determine how hard a player is to tackle. Kinetic energy measures the energy a player brings to the collision, due to his size and speed. Momnentum, which changes with every small change in direction, determines how hard they are to bring down. Dynamic, wriggly players are maximising the effects of their kinetic energy by skilfully changing their momentum.

Also, dynamic players add energy to the system using leg drive and body strength. A small player that drives into the collison area may have more kinetic energy than a big man who simply hits it. Hence, the Faingaas and Horans of the world.

 

[Scotty]

AAC (Adam Ashley-Cooper) and Ioane are the best example of this (leg drive) currently in Oz rugby. They leave most of the forwards for dead.

Fa'ainga probably needs to be a bit smarter about his crashball by changing the point of contact sometimes. He often just puts his head down and goes as hard as possible - not a bad trait, but he isn't always going to come out of it intact.

 

[Groucho]

Yes, AAC (Adam Ashley-Cooper)'s leg drive is phenomenal. Defenders have to tackle him multiple times.

 

[Groucho]

Bruce, we're mostly talking about billard balls here. Velocity (speed) is all that matters, not whether the ball is speeding up or down. But with your human units, I reckon the force of a person slowing up to 10 m/s will be less that someone speeding up to 10 m/s. The body changes shape in speeding up and slowing down.

That's absolutely right Scarfman. The equation I used was the simplest one, literally the kinetic energy of a rigid body. Also, in slowing down and speeding up you will get rotation as well as elasticity.

However there are equations to deal with all of those variables. Ultimately, a calculation of the energy required to stop a player based on the momentum in the system, and a calculation based on the kinetic energy in the system, must of course yield the same answer, since it is the same system.

 

[Reddy!]

Blah blah blah blah, skip to the point please! So who has more impact - Fainga'a or Carter? That's all I care about.

 

[Scotty]

Once a game, when Carter really uses his weight, then it is him. All other times it is Fa'ainga.

 

[cyclopath]

Well, Reddy!, I think the 2 players were used as examples of players of different masses, rather than a pissing competition for starters, but I suspect the answer would depend upon how fast either was running at the time...i.e. the point of the thread, I mean all that blah, blah, blah.

 

[Ash]

Well, theoretically, acceleration at the point of impact does not matter, if it is in a straight line. However, after the point of impact it may matter if the method of producing the acceleration (ie leg drive) can continue to be used by the player. But really, straight line accelerating and deceleration do not matter, only velocity at the point of impact in a theoretical model that Groucho is talking about. The exceptional problem that Groucho / Scarfman referred to is either modelling the impact in an ideal scenario in a minute quantum of time, or a more real world model where the impact takes place over a second.

Angular acceleration / deceleration, though, may be different (ie changing direction). I think this is what Groucho said above, where small guys changing their direction with no loss in speed have an advantage. By changing the point of impact, you can change how much force is required to stop you (that is where, no doubt, the biomechanics that Bruce is so interested come into play). Especially where the force is being applied (hips (close to the centre of gravity), chest, thighs, etc), or from what direction - eg if the force is normal to the guy running, or in the same direction, or coming from some weird angle, or maybe even what part of the stride/gait the player is in (eg driving off one foot during the initial collision might provide more stable force throughout the collision for the runner).

But we're now at the Eddie Jones level of analysis, I think.

 

[liquor box]

another factor to consider could be the size of the point of impact. Surely there would be more damage done by a smaller impact zone than a large one where the impact is more evenly spread.

A Fainga is smaller than Carter then you would imagine that any part of the body that makes impact would be smaller and theirfore cause more pain. This could also explain why small people with less weight can put on big hits.

Would you rather be hit by say A Faingas shoulder at 10m/s or Ma'afu gut at 10m/s? there would be a massive difference in the damage caused

 

[Groucho]

Yet another point to consider is that the defender doesn't have to supply all the energy required to stop the attacker himself.

Consider the case of a perfect bootlace tackle: a very small man can bring a very big man down in minimal space with the application of a tiny amount of force. In this case the defender is using the attacker's momentum to exploit the ground as a form of massive second defender, absorbing most of the attacker's kinetic energy. Fainga'a does a lot of this.

 

[Scotty]

Now you are talking about differential centre of gravity of the impacting forces. As Spewn alluded to above, gravity comes into play, and it probably comes into play more the higher the differential of the centres of gravity of these two forces.