Tres Juicy Posted January 10, 2012 Share Posted January 10, 2012 Hi all, How can I measure impact? I'd like to build something relatively simple that can measure the impact of a punch It needs to be fairly simple and portable, I thought about canibalising some bathroom scales... Any ideas? Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 10, 2012 Share Posted January 10, 2012 (edited) Low tech: Ballistic pendulum. High tech: There are 6DOF load cells designed for sports medicine that would likely excel at this when coupled with a suitable data acquisition system. Edited January 10, 2012 by InigoMontoya Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 10, 2012 Author Share Posted January 10, 2012 (edited) Ballistic pendulum:- Good idea, but how could I callibrate it so that I know how much movement equates to how much energy? I would need something that generates a known amount of energy High tech is too costly I'm afraid... Edited January 10, 2012 by Tres Juicy Link to comment Share on other sites More sharing options...
dimreepr Posted January 10, 2012 Share Posted January 10, 2012 Hi all, How can I measure impact? I'd like to build something relatively simple that can measure the impact of a punch It needs to be fairly simple and portable, I thought about canibalising some bathroom scales... Any ideas? maybe there's an app for the iphone, not looked as I avoid apple products like the plague. Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 11, 2012 Share Posted January 11, 2012 (edited) Ballistic pendulum:- Good idea, but how could I callibrate it so that I know how much movement equates to how much energy? I would need something that generates a known amount of energy A ballistic pendulum doesn't measure energy. It measures momentum. And the beauty of a ballistic pendulum is that there really isn't any calibration required (assuming accuracy of a few percent is acceptable). It's a first principles device... You just do the math. Edited January 11, 2012 by InigoMontoya Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 11, 2012 Author Share Posted January 11, 2012 A ballistic pendulum doesn't measure energy. It measures momentum. And the beauty of a ballistic pendulum is that there really isn't any calibration required (assuming accuracy of a few percent is acceptable). It's a first principles device... You just do the math. Yes, but momentum comes from the energy that you put in, so you can extrapolate the amount of energy used to cause that momentum. In terms of calibration what I'm asking is surely I would need to hit it with a known force to give me an idea of how much force is needed to produce X amount of movement? Or, are you saying that I should calculate the force required using the length and weight of the thing and produce a scale that way? As for smartphone apps - I'm not really willing to smash up my phone and I very much doubt they are any good Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 12, 2012 Share Posted January 12, 2012 Yes, but momentum comes from the energy that you put in, so you can extrapolate the amount of energy used to cause that momentum. Not really. If you were talking about a bullet or something? Sure, you could. But you're asking about a punch. With that in mind... What is the effective mass of the "projectile?" Is it just the hand? The arm? How much of the shoulder's mass would be involved? The torso? Much is going to depend on body build and technique. Thus, while you could in theory come up with some energy numbers, I would put precisely zero faith in them. In terms of calibration what I'm asking is surely I would need to hit it with a known force to give me an idea of how much force is needed to produce X amount of movement? Or, are you saying that I should calculate the force required using the length and weight of the thing and produce a scale that way? Are you familiar with the concept of a ballistic pendulum? Here, read the wiki article. There is no need to produce a scale. You just measure the mass of the pendulum and how high the pendulum swings. True, there's still the issue of the mass involved, but you don't really care. Mass is only needed if you want to back out a velocity. But if you're just interested in *impulse* (ie, momentum), simply using a heavy pendulum will minimize errors. Link to comment Share on other sites More sharing options...
ewmon Posted January 12, 2012 Share Posted January 12, 2012 A ballistic pendulum doesn't measure energy. It measures momentum. I was thinking that the maximum deflection in degrees can be equated with rise, nominally, 1-cosΘ, and that the rise is, of course, potential energy. Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 12, 2012 Author Share Posted January 12, 2012 Not really. If you were talking about a bullet or something? Sure, you could. But you're asking about a punch. With that in mind... What is the effective mass of the "projectile?" Is it just the hand? The arm? How much of the shoulder's mass would be involved? The torso? Much is going to depend on body build and technique. Thus, while you could in theory come up with some energy numbers, I would put precisely zero faith in them. In a decent punch you use as much of the bodyweight as possible behind it to increase the force of the impact, so the mass of the projectile will vary depending on technique (each punch will have an unknown quantity of mass behind it) If I were trying to measure the speed of the punch I would need to know the mass involved, but I'm not interested in speed only the force of the impact. Are you familiar with the concept of a ballistic pendulum? Here, read the wiki article. There is no need to produce a scale. You just measure the mass of the pendulum and how high the pendulum swings. True, there's still the issue of the mass involved, but you don't really care. Mass is only needed if you want to back out a velocity. But if you're just interested in *impulse* (ie, momentum), simply using a heavy pendulum will minimize errors. "You just measure the mass of the pendulum and how high the pendulum swings." Yes, but there must be a way (knowing the mass of the pendulum) to work out how much force is required to make it swing to a certain height? From this I could them produce a scale: Hieght A = Force X Hieght B = Force Y And so on... Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 12, 2012 Share Posted January 12, 2012 (edited) Yes, but there must be a way (knowing the mass of the pendulum) to work out how much force is required to make it swing to a certain height? No, that's not how a ballistic pendulum works. It doesn't measure force. It measures momentum (aka impulse). From this I could them produce a scale: Hieght A = Force X Hieght B = Force Y Nope. You could produce a scale that says... Height A = Impulse A Height B = Impulse B etc. ....But that's not the same thing as force. Force. Momentum. Apples. Oranges. Edited January 12, 2012 by InigoMontoya Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 12, 2012 Author Share Posted January 12, 2012 No, that's not how a ballistic pendulum works. It doesn't measure force. It measures momentum (aka impulse). Nope. You could produce a scale that says... Height A = Impulse A Height B = Impulse B etc. ....But that's not the same thing as force. Force. Momentum. Apples. Oranges. I think we may have been caught up in the semantics here (Force/impulse)... What I'm asking is can I use this to measure the impact energy? (whether you choose to call it the "force" of the impact or the "impulse" energy of the impact) Link to comment Share on other sites More sharing options...
Xittenn Posted January 12, 2012 Share Posted January 12, 2012 Height 100% = Oh Baby Height 90% = You are so strong pappy Height 10% = My baby can hit harder than you 1 Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 12, 2012 Author Share Posted January 12, 2012 Height 100% = Oh Baby Height 90% = You are so strong pappy Height 10% = My baby can hit harder than you Not the most scientific of approaches.... Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 13, 2012 Share Posted January 13, 2012 I think we may have been caught up in the semantics here (Force/impulse)... What I'm asking is can I use this to measure the impact energy? (whether you choose to call it the "force" of the impact or the "impulse" energy of the impact) Well, you could argue that it's semantics but... "Energy" has a very specific meaning in engineering. "Force" has a very specific meaning in engineering. "Impulse" has a very specific meaning in engineering. You've been trying to use the words somewhat interchangably, but none of those three words mean the same thing and they can not be used interchangably in an engineering context. So.... Could you use a ballistic pendulum to measure impact energy? Not without knowing the precise mass (which we don't). Could you use a ballistic pendulum to measure impact force? No. Could you use a ballistic pendulum to measure impact impulse? Yes. 1 Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 13, 2012 Share Posted January 13, 2012 I was thinking that the maximum deflection in degrees can be equated with rise, nominally, 1-cosΘ, and that the rise is, of course, potential energy. Missed this post until just now.... True, the rise is potential energy and it would provide a minimum value, but the energy contained in a punch is not likely to be efficiently converted to potential energy in the pendulum. Significant amounts are going to be lost to things like deformation of the tissues of the hand/arm. You know, watch high speed video of a punch. See all the waves moving through the flesh? That represents energy that will not show up in your pendulum. In other words, while energy IS conserved, that doesn't mean it is effectively converted to kinetic/potential energy within the pendulum. Momentum, on the other hand, *is* effectively converted. Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 13, 2012 Author Share Posted January 13, 2012 Well, you could argue that it's semantics but... "Energy" has a very specific meaning in engineering. "Force" has a very specific meaning in engineering. "Impulse" has a very specific meaning in engineering. You've been trying to use the words somewhat interchangably, but none of those three words mean the same thing and they can not be used interchangably in an engineering context. So.... Could you use a ballistic pendulum to measure impact energy? Not without knowing the precise mass (which we don't). Could you use a ballistic pendulum to measure impact force? No. Could you use a ballistic pendulum to measure impact impulse? Yes. I'm not an engineer, Can it be done? What will the impact impulse tell me? Link to comment Share on other sites More sharing options...
InigoMontoya Posted January 13, 2012 Share Posted January 13, 2012 What will the impact impulse tell me? It will tell you how fast the other guy's head will snap back when you hit it. Link to comment Share on other sites More sharing options...
Tres Juicy Posted January 13, 2012 Author Share Posted January 13, 2012 It will tell you how fast the other guy's head will snap back when you hit it. That's a start... Link to comment Share on other sites More sharing options...
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