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Please, for crying out loud, if you've any doubt of the 'momentum' aspect of running and passing, watch this [url=http://www.youtube.com/watch?v=RgMlDy2jP9sVideo[/url explaining exactly how it works. It's a RU video, but the concept is the same. Some of the passes are a bit iffy but look past that and it should all become clear.
If the link gets removed, look for "Rugby Union "Forward Pass" video".
Simples.
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| Quote ="Cronus"Please, for crying out loud, if you've any doubt of the 'momentum' aspect of running and passing, watch this [url=http://www.youtube.com/watch?v=RgMlDy2jP9sVideo[/url explaining exactly how it works. It's a RU video, but the concept is the same. Some of the passes are a bit iffy but look past that and it should all become clear.
If the link gets removed, look for "Rugby Union "Forward Pass" video".
Simples.'"
Yeah, I've seen that video before. It's a good experiment badly performed though. Some of them passes are actually released in a forward direction!
I'll further add to that that the "computer simulation" would never occur, as the ball would never travel straight, it would be curved due to acceleration. |
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| Quote ="Wellsy13"Yeah, I've seen that video before. It's a good experiment badly performed though. Some of them passes are actually released in a forward direction!
I'll further add to that that the "computer simulation" would never occur, as the ball would never travel straight, it would be curved due to acceleration.'"
Like I say, some of the passes are 'iffy' at best - I certainly wouldn't look at the hands, but then those are Union players.
But for those who are somehow struggling to understand that if you're running and you pass to a player slightly behind you, the ball will actually move forwards from its original start point while still travelling backwards relative to you and the receiver, it's not a bad guide. |
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"Momentum is mass times velocity - for the purposes of the discussion, they're pretty interchangable''
Do you really believe that if an eighteen-stone prop forward and a ten-stone scrum half are running at the same velocity and they both throw a ball in the same direction at the same speed then the one from the prop forward will follow a different trajectory than the one from the scrum half? The original J. Willard Gibbs would not agree with you.
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I'm back for more abuse
All the physics equations so far are incomplete. They all infer that the players body is still facing the opponents line at the point of release. This is never the case. The player usually changes his angle at time of pass and at the same time rotates his upper body towards the direction he intends to pass adding/shifting mass/velocity. Whilst I realise this is what creates the backward force I think we are possible trivialising its impact and certainly defeats the previous argument about throwing a ball from a moving pickup truck.
Wellsy-please get on to boots n all. Would love to see Stevo justyfying himself and Clarkey trying to pretend he understands.
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It's simple enough. Think relativity rather than momentum though. That's what the rule book says.
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| Quote ="JBS"Do you really believe that if an eighteen-stone prop forward and a ten-stone scrum half are running at the same velocity and they both throw a ball in the same direction at the same speed then the one from the prop forward will follow a different trajectory than the one from the scrum half? The original J. Willard Gibbs would not agree with you.'"
What are you on about? You're the one who said:
[iCould we please stop talking about the 'momentum' rule. The player's momentum is irrelevant. The quantity that affects the trajectory of the ball is his velocity[/i
For any argument you can make with velocity, momentum is just as useful, as the mass of the player and ball remain basically constant during motion.
If a players momentum is irrelevant, then so is his velocity - as that velocity is [idirectly[/i related to momentum, it's just a simple multiplication.
Edit:
Sorry, I'm not trying to be a pain here - you're quite right in pointing out that the "momentum rule" is a bit of a misnomer, and such things are far better dealt with in terms of the velocity. |
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| Quote ="JBS""Momentum is mass times velocity - for the purposes of the discussion, they're pretty interchangable''
Do you really believe that if an eighteen-stone prop forward and a ten-stone scrum half are running at the same velocity and they both throw a ball in the same direction at the same speed then the one from the prop forward will follow a different trajectory than the one from the scrum half? The original J. Willard Gibbs would not agree with you.'"
It's been a while since I did biomechanics so this may be wrong, but I think the mass in question isn't the mass of the player thowing the projectile (i.e. the ball) but the mass of the projectile itself. The player with the ball is just adding velocity in a given direction to the ball.
Basically, if two people were throwin two different weighted balls in the same direction at the same velocity they would flow on different paths due to the momentum being different, so your point about the masses of the players themselves would be irrelevant I think (I may need to look that up, but I think it's right).
This doesn't differ from the original point though about players running at different velocities (i.e. one being static and one being on the move) as obviously the velocity would be different and the mass the same, thus a different path. |
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| Quote ="Lost in Leeds"I'm back for more abuse
All the physics equations so far are incomplete. They all infer that the players body is still facing the opponents line at the point of release. This is never the case. The player usually changes his angle at time of pass and at the same time rotates his upper body towards the direction he intends to pass adding/shifting mass/velocity. Whilst I realise this is what creates the backward force I think we are possible trivialising its impact and certainly defeats the previous argument about throwing a ball from a moving pickup truck.
Wellsy-please get on to boots n all. Would love to see Stevo justyfying himself and Clarkey trying to pretend he understands.'"
We aren't abusing you (well, I aren't anyways!) so don't worry! All disagreements don't have to be abusive!
They don't infer anything about the bodies direction, they just infer the velocity given by the body to the ball in a given direction. The body may change which way it is facing, but the direction the body is going in is still the same. The velocity may change due to turning, but the fact that there is velocity there in the given direction still has an affect on the direction the ball will travel in. If you ran forward with a ball and turned the body and passed, the ball will travel in a different direction to if you stood still in the exact same position as you threw the ball.
Perhaps a better example of this would be if you ran at the exact same speed on a treadmill and threw the ball in the same direction. The ball would not travel forward as the velocity would be zero (despite running, you aren't actually moving, thus the ball will be getting no extra forward velocity and thus not effecting its path).
I've emailed Boots N All last night. When I get home I'll post the email on here if I have time. Would be a very interesting segment, mainly to see if they can get it right! |
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| Quote ="Wellsy13"
Basically, if two people were throwin two different weighted balls in the same direction at the same velocity they would flow on different paths due to the momentum being different, so your point about the masses of the players themselves would be irrelevant I think (I may need to look that up, but I think it's right).'"
Not quite - even though the heavier ball would require more force to get it to the same velocity, it would still follow the same path as the lighter ball. Mass would have no effect on speed, direction, or the path of the ball under any non-relativistic circumstances, regardless of how fast the inertial frame of reference (the passer) was moving - mass would only effect the required force to get it [ito[/i that speed and in that direction. |
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| Ignoring air resistance!  |
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| Quote ="J. Willard Gibbs"Ignoring air resistance!
'"
Which would cause a force on the ball, and as I said, the mass only effects the force, so I took that into account |
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Quote ="Wheres My Shirt"Which would cause a force on the ball, and as I said, the mass only effects the force, so I took that into account
'"
"Mass would have no effect on speed, direction, or the path of the ball"
It's not as straightforward as you might think; if the two balls have the same shape and velocity at release, they don't necessarily have the same trajectory ...
Let's ignore gravity and just look at the air resistance - the force from that air resistance is proportional to the velocity squared (roughly speaking):
F = kv^2 (k is some friction factor)
We're expecting the force due to air resistance to be the same on both balls, as it only depends on the shape of the ball and the velocity. As we all know, F = ma, and so;
kv^2 = ma
kv^2 = m d/dt v
or, another way,
k (dr/dt)^2 = m d^r/dt^2
So we see that how the velocity of the ball changes in the face of air resistance contains a term dependent on the mass of the ball.
I suspect that the mass of the ball is therefore significant in the case of 2 otherwise identical balls thrown at the same speed in the presence of air resistance. I think the trajectories may well be different, but I've not bothered to plot any of them!
Edit - rather than bothering to solve the equations of motion numerically, I used some Google-fu and came up with this:
jersey.uoregon.edu/vlab/Cannon/
which I think demonstrates the problem. Click the drag on button, and fire the cannon. then only change the density of the projectile, and try again! |
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Quote ="Wheres My Shirt"Which would cause a force on the ball, and as I said, the mass only effects the force, so I took that into account
'"
"Mass would have no effect on speed, direction, or the path of the ball"
It's not as straightforward as you might think; if the two balls have the same shape and velocity at release, they don't necessarily have the same trajectory ...
Let's ignore gravity and just look at the air resistance - the force from that air resistance is proportional to the velocity squared (roughly speaking):
F = kv^2 (k is some friction factor)
We're expecting the force due to air resistance to be the same on both balls, as it only depends on the shape of the ball and the velocity. As we all know, F = ma, and so;
kv^2 = ma
kv^2 = m d/dt v
or, another way,
k (dr/dt)^2 = m d^r/dt^2
So we see that how the velocity of the ball changes in the face of air resistance contains a term dependent on the mass of the ball.
I suspect that the mass of the ball is therefore significant in the case of 2 otherwise identical balls thrown at the same speed in the presence of air resistance. I think the trajectories may well be different, but I've not bothered to plot any of them!
Edit - rather than bothering to solve the equations of motion numerically, I used some Google-fu and came up with this:
jersey.uoregon.edu/vlab/Cannon/
which I think demonstrates the problem. Click the drag on button, and fire the cannon. then only change the density of the projectile, and try again! | |
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Quote ="J. Willard Gibbs"icon_biggrin.gif
"Mass would have no effect on speed, direction, or the path of the ball"
It's not as straightforward as you might think; if the two balls have the same shape and velocity at release, they don't necessarily have the same trajectory ...
Let's ignore gravity and just look at the air resistance - the force from that air resistance is proportional to the velocity squared (roughly speaking):
F = kv^2 (k is some friction factor)
We're expecting the force due to air resistance to be the same on both balls, as it only depends on the shape of the ball and the velocity. As we all know, F = ma, and so;
kv^2 = ma
kv^2 = m d/dt v
or, another way,
k (dr/dt)^2 = m d^r/dt^2
So we see that how the velocity of the ball changes in the face of air resistance contains a term dependent on the mass of the ball.
I suspect that the mass of the ball is therefore significant in the case of 2 otherwise identical balls thrown at the same speed in the presence of air resistance. I think the trajectories may well be different, but I've not bothered to plot any of them!
Edit - rather than bothering to solve the equations of motion numerically, I used some Google-fu and came up with this:
jersey.uoregon.edu/vlab/Cannon/
which I think demonstrates the problem. Click the drag on button, and fire the cannon. then only change the density of the projectile, and try again!'"
Very handy little tool that. The RFL should invest in one to demonstrate passing! 
The factors could be velocity, angle of release, wind resistance and ball size (well there's a size 3, 4 and 5 ball isn't there!). That'd be good in their resources section!
That tool shows that I am right that the weight matters, but wrong that it is because of momentum (taking the drag out means the cannon ball still travels in the same trajectory regardless of density).
What is it the mass of the player then? Or does the ball not travel forwards due to momentum, but due to something else? Wish this thread had been this time last year when I was doing the module as all the notes would be right in front of me! |
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Quote ="J. Willard Gibbs"icon_biggrin.gif
"Mass would have no effect on speed, direction, or the path of the ball"
It's not as straightforward as you might think; if the two balls have the same shape and velocity at release, they don't necessarily have the same trajectory ...
Let's ignore gravity and just look at the air resistance - the force from that air resistance is proportional to the velocity squared (roughly speaking):
F = kv^2 (k is some friction factor)
We're expecting the force due to air resistance to be the same on both balls, as it only depends on the shape of the ball and the velocity. As we all know, F = ma, and so;
kv^2 = ma
kv^2 = m d/dt v
or, another way,
k (dr/dt)^2 = m d^r/dt^2
So we see that how the velocity of the ball changes in the face of air resistance contains a term dependent on the mass of the ball.
I suspect that the mass of the ball is therefore significant in the case of 2 otherwise identical balls thrown at the same speed in the presence of air resistance. I think the trajectories may well be different, but I've not bothered to plot any of them!
Edit - rather than bothering to solve the equations of motion numerically, I used some Google-fu and came up with this:
jersey.uoregon.edu/vlab/Cannon/
which I think demonstrates the problem. Click the drag on button, and fire the cannon. then only change the density of the projectile, and try again!'"
Very handy little tool that. The RFL should invest in one to demonstrate passing!
The factors could be velocity, angle of release, wind resistance and ball size (well there's a size 3, 4 and 5 ball isn't there!). That'd be good in their resources section!
That tool shows that I am right that the weight matters, but wrong that it is because of momentum (taking the drag out means the cannon ball still travels in the same trajectory regardless of density).
What is it the mass of the player then? Or does the ball not travel forwards due to momentum, but due to something else? Wish this thread had been this time last year when I was doing the module as all the notes would be right in front of me! | |
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| Quote ="J. Willard Gibbs"icon_biggrin.gif
So we see that how the velocity of the ball changes in the face of air resistance contains a term dependent on the mass of the ball.
'"
It does, but I also said that the mass only effects the required force, the part you quoted was turning a bit of a blind eye to that - the fact air resistance causes a force is pretty meaningless in a theoretical (and tedius) sense - between the ball hitting each particle of air the mass would have no effect on the speed, direction, or path. A force causing balls of different masses to decelerate at different rates is not contrary to what I intially stated - indeed it is Newton's 2nd law.
Essentially, air resistance is a force and so mass would effect that. I said that.
The mass of the player is thoroughly irrelevant. If, for example, the player dropped the ball vertically as he ran forward, the ball would have gained its forward velocity due to the initial force required to accelerate the player (and ball) to that velocity. Once at that velocity it will maintain it forever unless other forces are applied. |
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| Quote ="Wheres My Shirt"The mass of the player is thoroughly irrelevant. If, for example, the player dropped the ball vertically as he ran forward, the ball would have gained its forward velocity due to the initial force required to accelerate the player (and ball) to that velocity. Once at that velocity it will maintain it forever unless other forces are applied.'"
Right, so what we've learned here is that...
Momentum, i.e. mass x velocity, isn't relevant to why the ball travels forward.
Mass is only relevant in relation to the ball, and that only matters in relation to forces acting upon it (i.e. the player, air resistance and gravity).
Velocity is the only important factor in why the ball travels forward. That is, the velocity of the player in a given direction is transferred to the ball at the time of release, and it's path will be defined by that in conjunction with: the angle of release and the force applied to the ball at that angle of release (as well as the forces of gravity and air resistance).
So really, momentum appears to have absolutely nothing to do it. Stevo should rename it to a more accurate "transferable velocity" rule! |
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| Seriously though, you all need to get laid.
Or have a damn good monkey spank.
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| What Wellsy said on the last page was actually correct:
Quote ="Wellsy"Basically, if two people were throwin two different weighted balls in the same direction at the same velocity they would flow on different paths due to the momentum being different'"
But you didn't think so:
Quote ="you"Not quite - even though the heavier ball would require more force to get it to the same velocity, it would still follow the same path as the lighter ball'"
I just pointed out that your refutation was only valid in the case of no air resistance. Your references to the force required was purely i the context of getting the ball up to that velocity as you passed it, not what happened afterwards:
Quote ="you"Mass would have no effect on speed, direction, or the path of the ball under any non-relativistic circumstances'"
Which is not true in the case of air resistance being present. |
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| Quote ="Wellsy13"So really, momentum appears to have absolutely nothing to do it. Stevo should rename it to a more accurate "transferable velocity" rule!'"
Absolutely, momentum would only really apply in a perfect elastic reaction - whereby the momentum of the player is transferred to the ball. Considering the player has about 500 kgm/s, that would mean a player would need to throw the ball at about mach 3 for momentum to have any application. |
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| Quote ="Wellsy13"And I suppose you're getting it as you type!
'"
I'm in post-coital internet mode. As in, she's up earlier than me so she's asleep.
Quote ="Wellsy13"Did nobody tell you that chicks dig an educated bloke
'"
I'm educated. I can count to 120 before I finish, roll over and go to sleep. |
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| Quote ="Cronus"I'm educated. I can count to 120 before I finish, roll over and go to sleep.'"
That's quite impressive! I get bored after about 22 |
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| Quote ="J. Willard Gibbs"
I just pointed out that your refutation was only valid in the case of no air resistance. Your references to the force required was purely i the context of getting the ball up to that velocity as you passed it, not what happened
'"
Not true, I actually changed my wording so that that would not be the case - "mass would only effect the required force to get it to that speed and in that direction".
Just because I didn't make clear what was applying the force doesn't change the fact I didn't exclude air resistance. |
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Can you work out the path of the ball then (other forces acting on it like air resistance and gravity) using trigonometry then?
i.e. If a static player passes a ball backwards at a 45degree angle at 10m/s, then using trigonometry it's velocity in a directly backwards direction is 7m/s.
If then a second player makes the exact same pass but is running forwards at 10m/s, then it would travel forward at 3m/s? (10m/s forwards - 7m/s backwards = 3m/s forwards).
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| Quote ="Wellsy13"Can you work out the path of the ball then (other forces acting on it like air resistance and gravity) using trigonometry then'"
Using Newtonian physics you can, yes.
To your example, then yes, the ball would travel forward at approximately 3m/s. At such slow speeds air resistance wouldn't have too much of an effect anyway. |
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