# Thread: Calculations of Momentum P=mv, and Kinetic energy KE=(mv^2)/2 of a racket on shuttle

1. Originally Posted by craigandy
No, you can too easily get a bad reading because who knows where on the racket string bed the shuttle came off or if their was slight slice during the test. Maybe a few mm off center makes no difference to feel or sound but big enough difference to speed. I just don't know.
Eh? Direct measurement of shuttle is surely more accurate and... direct, no? After all you're just guesstimating what the "c" is in L&L's formula.
Last edited by visor; 06-01-2013 at 03:29 PM.

2. Originally Posted by visor
Eh? Direct measurement of shuttle is surely more accurate and... direct, no? After all you're just guesstimating what the "c" is in L&L's formula.
Imo for comparisons of rackets I think ignore real shuttle speed and calculate hypothetical. I think this would be more accurate to work out the rackets capeabilty in your hands. If you could get a camera to record from the front as well and some sort of sound device so you could tell if you have peached it out the middle and then only use those results, then then Shuttle speed would be more accurate for comparison, yes!
Guesstimating the "c" does not make a notable difference when comparing different weighted rackets as I would imagine the "c" would be the same regardless of racket weight so it's the same difference it won't hurt the formula for comparisons.

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Originally Posted by craigandy
No, you can too easily get a bad reading because who knows where on the racket string bed the shuttle came off or if their was slight slice during the test. Maybe a few mm off center makes no difference to feel or sound but big enough difference to speed. I just don't know.
If you're doing this as a practical test, then of course you measure the shuttle speed!

common technique is to measure a decent sized sample, remove outliers, then obtain the average. Do for both rackets and compare.

Then it doesn't matter about slight slices etc as the effects will be the same (averaged) in both data sets.

What you are left with is average smash shuttle speed for your rackets.

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Originally Posted by craigandy
I think the model formula that line and length came up with is maybe not incredibly accurate for the actual speed of the shuttle but I see no reason why it is not an excellent formula for comparing rackets.
that's terrible logic. If a model isn't accurate then it is missing some physics. So why use it to compare that which it does not model well? Crazy.

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But if it's proportional to the correct number, then it's just as valid as a comparison.

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Originally Posted by SolsticeOfLight
But if it's proportional to the correct number, then it's just as valid as a comparison.
you've just assumed that the model is accurate in that statement so it's a bit of a self fulfilling sentence.

7. Originally Posted by amleto
that's terrible logic. If a model isn't accurate then it is missing some physics. So why use it to compare that which it does not model well? Crazy.
I understand why you think it's bad logic. But I am under the assumption that the model is only inaccurate because of the estimation of the "cor" which is fine, ergo perfect logic. It seems to model really well, I don't understand for comparisons of rackets what could be missing.

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But I am under the assumption that the model is only inaccurate because of the estimation of the "cor" which is fine, ergo perfect logic
Under what basis can you assume that? The model has not been validated at all.

The fudge factors at the moment are CoR estimates - total guesses, and also the racket head mass that is being used - total guess.

There is not a clear defined method of obtaining the mass used in the calculations from the actual mass of the racket.

Since you have not been recording the masses and BPs of the rackets, you have no idea if this model is more or less accurate when trying to predict shuttle velocity for rackets with hi/medium/low BPs.

I think a lesson in The scientific Method might be needed

It seems to model really well, I don't understand for comparisons of rackets what could be missing.
How can you say that when there has been no comparison at all done between theory prediction and empirical evidence? There has only been 'well, it's in the ball park' hand waving. We have covered elsewhere that there is a lot of physics missing from this linear approximation of a badminton swing & shuttle contact.
Last edited by amleto; 06-02-2013 at 05:41 PM.

9. Originally Posted by amleto
Under what basis can you assume that? The model has not been validated at all.

The fudge factors at the moment are CoR estimates - total guesses, and also the racket head mass that is being used - total guess.

There is not a clear defined method of obtaining the mass used in the calculations from the actual mass of the racket.

Since you have not been recording the masses and BPs of the rackets, you have no idea if this model is more or less accurate when trying to predict shuttle velocity for rackets with hi/medium/low BPs.

I think a lesson in The scientific Method might be needed
Yeah you are right, The whole mass thing does need to be defined properly, that is a given(can you help by doing an example of method specific to the situation?). The cor I am not so concerned about although i will try doing that on the software hitting shuttles into a clamped racket.

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the only thing that I can think of to calculate what mass to use for the head in the linear approximation is to use something like visor's pseudo swing weight, but instead of having the weight at the tip of the frame, consider the weight at the centre of the head.

11. Just a thought can we work out the head weight that includes the BP by using/plotting the speeds if we anchor the handle and drop the racket so it swings like a pendulum. Is this crazy? is there a formula for it?

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T=2.pi.sqrt(I/[m.g.bp])

See pendulum wiki page.
http://en.wikipedia.org/wiki/Pendulum

Really means you need to measure the bp an then thiswill give you I (MoI)

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Originally Posted by amleto
the only thing that I can think of to calculate what mass to use for the head in the linear approximation is to use something like visor's pseudo swing weight, but instead of having the weight at the tip of the frame, consider the weight at the centre of the head.
I'm going to disagree with myself and say that this is incorrect.

14. Originally Posted by amleto
T=2.pi.sqrt(I/[m.g.bp])

See pendulum wiki page.
http://en.wikipedia.org/wiki/Pendulum

Really means you need to measure the bp an then thiswill give you I (MoI)
Ok thanks and how does this give me the head mass?

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This is a very good thread. Maybe, we can use it to solve my problem. Pls help.
I have 2 rackets, A & B. Both have the same length (675cm), stiffness (medium flex), string type & string tension. The difference is on the weight (wet weight, string & grip included).
The weight of racket A is 98 grams, racket B is 94 grams.
I set both racket at the same bp=290mm, when I use racket A at first then switch to racket B at the next game, I can feel that I need more energy to push the shuttle when doing dropshot using racket B. If I use the same effort (energy) like when using racket A, the shuttle will meet the net.
Maybe, we can solve this by increasing the bp of racket B (usually by cutting the grip shorter). The question is: what is the bp of racket B so it can substitude racket A perfectly?
Thank you...

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Originally Posted by craigandy
Ok thanks and how does this give me the head mass?
sorry, it does not. I cannot think of a non-arbitrary non-empirical method that will give you the mass to use for the linear approximation case.

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Originally Posted by todjo
This is a very good thread. Maybe, we can use it to solve my problem. Pls help.
I have 2 rackets, A & B. Both have the same length (675cm), stiffness (medium flex), string type & string tension. The difference is on the weight (wet weight, string & grip included).
The weight of racket A is 98 grams, racket B is 94 grams.
I set both racket at the same bp=290mm, when I use racket A at first then switch to racket B at the next game, I can feel that I need more energy to push the shuttle when doing dropshot using racket B. If I use the same effort (energy) like when using racket A, the shuttle will meet the net.
Maybe, we can solve this by increasing the bp of racket B (usually by cutting the grip shorter). The question is: what is the bp of racket B so it can substitude racket A perfectly?
Thank you...
Can't be determined precisely with only the information given.

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