"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.
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.
QuoteJBS="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.
QuoteJBS="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.
QuoteLost in Leeds="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!
QuoteWellsy13="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.
QuoteWheres My Shirt="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:
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!
QuoteWheres My Shirt="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:
"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:
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!
QuoteJ. Willard Gibbs="J. Willard Gibbs":D
"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:
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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