bhaazee Posted January 23, 2012 Posted January 23, 2012 Hello Science Forums, I have a car travelling a certain velocity and absorbs x Joule of Kinetic energy on impact. I don't have the time duration of impact. I need to calculate the force at which the car made the impact. Can someone suggest me on how to find the force? Regards
DrRocket Posted January 23, 2012 Posted January 23, 2012 Hello Science Forums, I have a car travelling a certain velocity and absorbs x Joule of Kinetic energy on impact. I don't have the time duration of impact. I need to calculate the force at which the car made the impact. Can someone suggest me on how to find the force? Regards You can't without more information. A LOT more information. To get a force-time curve you would need to know, in gory detail, the properties of each and every material in the car and the specific geometry of each and every part. Even then it would require a very complex computer code to handle all of the dynamics and material response calculations. The guts of the problem is the time duration that you don't have, but to be absolutely correct the force will not be constant over any period of time, so the time duration would only allow you to calculate the time-averaged force, if you calculated the mass (which with velocity gives you momentum which is force x time ) from the given kinetic energy.
baxtrom Posted February 17, 2012 Posted February 17, 2012 You can't without more information. A LOT more information. To get a force-time curve you would need to know, in gory detail, the properties of each and every material in the car and the specific geometry of each and every part. Even then it would require a very complex computer code to handle all of the dynamics and material response calculations. The guts of the problem is the time duration that you don't have, but to be absolutely correct the force will not be constant over any period of time, so the time duration would only allow you to calculate the time-averaged force, if you calculated the mass (which with velocity gives you momentum which is force x time ) from the given kinetic energy. ..or you put on your engineering goggles, approximate the distance traveled by car center of mass during impact and then compute the average force during the impact event as [math]F = \frac{\textrm{kinetic energy}}{\textrm{distance}}[/math].
swansont Posted February 17, 2012 Posted February 17, 2012 ..or you put on your engineering goggles, approximate the distance traveled by car center of mass during impact and then compute the average force during the impact event as [math]F = \frac{\textrm{kinetic energy}}{\textrm{distance}}[/math]. Which still requires more information, since you don't know what the distance is.
John Cuthber Posted February 17, 2012 Posted February 17, 2012 About a quarter or half the length of the car- which is good enough to give you an order of magnitude or so for the force.
baxtrom Posted February 17, 2012 Posted February 17, 2012 Which still requires more information, since you don't know what the distance is. Which can be assumed by means of engineering judgement. For example, a typical car is 4-5 m long. The COG is going to be close to the middle of the car, i.e. perhaps at 2 m from the front. It is therefore a reasonable assumption that the distance traveled by the COG will not be greater than 2 m even for high energy crashes. Sure, you could do it the hard way. Explicit FEA solvers like LS-DYNA are available for complex impact analysis. Nonlinear material models and contact conditions put into a million-or-so DOF model would do the trick. I do not believe however that is the answer the TS is looking for.
DrRocket Posted February 17, 2012 Posted February 17, 2012 (edited) Which can be assumed by means of engineering judgement. For example, a typical car is 4-5 m long. The COG is going to be close to the middle of the car, i.e. perhaps at 2 m from the front. It is therefore a reasonable assumption that the distance traveled by the COG will not be greater than 2 m even for high energy crashes. Sure, you could do it the hard way. Explicit FEA solvers like LS-DYNA are available for complex impact analysis. Nonlinear material models and contact conditions put into a million-or-so DOF model would do the trick. I do not believe however that is the answer the TS is looking for. I have seen all sort or total nonsense assumed under the heading of "engineering judgement". I have had to overrule it, and in the process saved many milliions of dollars, and likley a life or two. Your approach is just more nonsense. There is nothing in the problem statement to indicate that the car meets an "immovable object", and hence the distance traveled by the CG need not be bounded by any reasonable function of the length of the car. Moreover, even if the car were to collide with the proverbial immovable object, the net force on the car will hardly be constant, so an average of force over distance traveled by the CG and an average over time will be totally different things. Better take the hard way. Silly solution based on unfounded and unrealistic assumptions is often much worse than simply admitting that you don't know. If you want to get fancy then maybe you even need a hydro code (e.g. Epic) and high-rate material properties. Been there, done that. Edited February 17, 2012 by DrRocket -1
swansont Posted February 17, 2012 Posted February 17, 2012 You don't know how much energy was transferred. It could be a car hitting a truck or a balloon or somewhere in between. You don't have enough information to solve the problem.
baxtrom Posted February 17, 2012 Posted February 17, 2012 I have seen all sort or total nonsense assumed under the heading of "engineering judgement". I have had to overrule it, and in the process saved many milliions of dollars, and likley a life or two. Your approach is just more nonsense. Silly solution based on unfounded and unrealistic assumptions is often much worse than simply admitting that you don't know. Oh, my. Scienceforum's own Professor Snape is at it again. Not that I didn't see it coming, just fun to watch the fireworks. No, the approach is not nonsense and deep inside you know it, but you have to defend your position and try to be the big bad bully of the forum. Really, I don't mind, it's sort of entertaining. A secret tip: don't fly, or even go by car or train, because design methods and codes are based on empirical observations and engineering judgement. Better stay in your cave scribling those integral equations on the walls.
DrRocket Posted February 17, 2012 Posted February 17, 2012 A secret tip: don't fly, or even go by car or train, because design methods and codes are based on empirical observations and engineering judgement. Better stay in your cave scribling those integral equations on the walls. Please inform me as to anything, anything at all, that has your design imprint, and I will most certainly try to avoid it. 1
baxtrom Posted February 17, 2012 Posted February 17, 2012 You don't know how much energy was transferred. It could be a car hitting a truck or a balloon or somewhere in between. You don't have enough information to solve the problem. I don't know which "energy transfer" you mean but the energy absorbed during impact is known according to the TS. The distance traveled by the car during impact is not stated in the original post but I believe I was quite clear in pointing out that the time average of the force could be approximated from a reasonable guess of this parameter. Yes, that is an assumption, and I thought it was evident from my post. Please inform me as to anything, anything at all, that has your design imprint, and I will most certainly try to avoid it. Stay away from civilization, that's my best tip honeybunny!
DrRocket Posted February 17, 2012 Posted February 17, 2012 Stay away from civilization, that's my best tip honeybunny! As you suggest, there are indeed lots of shoddy goods and poor designs on the market. But are they predominantly yours ? I wouldn't have guessed that. I don't know which "energy transfer" you mean but the energy absorbed during impact is known according to the TS. The distance traveled by the car during impact is not stated in the original post but I believe I was quite clear in pointing out that the time average of the force could be approximated from a reasonable guess of this parameter. Yes, that is an assumption, and I thought it was evident from my post. The time average of the force (presumably we are talking about the contact force between the car and whatever it hits) is tied to momentum, not energy. Better use a little engineering judgment and go read a physics book. Conservation of momentum will be a helpful principle in this problem. Conservation of energy is not so helpful as there are a lot mechanisms for dissipation of energy -- including material failure in the vehicle, which is a significant design objective in modern vehicle design -- that one cannot estimate without detailed knowledge of the car design and the pramaters of the collision. To make use of energy considerations you are right back to those detailed non-linear mechanics models.
John Cuthber Posted February 18, 2012 Posted February 18, 2012 "The time average of the force (presumably we are talking about the contact force between the car and whatever it hits) is tied to momentum, not energy. " The mass and velocity are probably known. It would certainly be odd not to have a fair idea of the mass. So the relation between energy and momentum is know, so... It's like that absurd idea where people say "it's not the voltage that kills, it's the current." Well, since one is directly proportional to, and not independent of, the other... "Conservation of momentum will be a helpful principle in this problem." I doubt that.
Xittenn Posted February 18, 2012 Posted February 18, 2012 ..or you put on your engineering goggles, approximate the distance traveled by car center of mass during impact and then compute the average force during the impact event as [math]F = \frac{\textrm{kinetic energy}}{\textrm{distance}}[/math]. The question is posed as a mechanics question in the context of engineering, and given the amount of information, there is no value that would have any meaning what so ever. If you do just as you have proposed what exactly is the significance of this value? I don't see one and so I am asking you, what do you suppose the value of this ascertained F that you are proposing is? Can you further apply it to any of this physical process in such a way as to derive any meaningful information from it what so ever? If we want to relate this figure back to the passenger the outcome will be that we are applying an average force over some time, but what about the sharp forces that are going to be felt at say t=.1s? Are these suddenly benign? Is this going to tell us anything about the stopping distance or the crumple zone based on the x Joules absorbed? No, because these are suddenly eliminated the second you made presumptions about a system that doesn't even truly exist, even in the slightest fashion; you told it what it was! So F is, who cares, and that is what I get from that! I recently posted a question in homework where I similarly told the physical situation what it was and it cost me .10% of my final grade; big lesson learned. What are these magic goggles that I have never seen before? What is the meaning of life, well 42 obviously, right? I will gladly be proven wrong on this as it would serve to simplify my life stupendously i.e. I want you to be right! -1
DrRocket Posted February 18, 2012 Posted February 18, 2012 (edited) "Conservation of momentum will be a helpful principle in this problem." I doubt that. Doubt anything that blows your skirt up. But you are wrong. Force acting over time is directly relatable to momentum. Force acting over distance is directly relatable to time. When your question involves force and time the relevant quantity is momentum rather than enrgy. It's like that absurd idea where people say "it's not the voltage that kills, it's the current." Well, since one is directly proportional to, ... Again this is only true for a linear resistive load. It is in general false. Edited February 18, 2012 by DrRocket
Xittenn Posted February 18, 2012 Posted February 18, 2012 (edited) There is a car traveling at velocity [math] \nu [/math], it absorbs x Joules of Kinetic Energy on impact. The value of time t is not known, what is the force F? a) 42 b) [math] \frac{\textrm{kinetic energy}}{\textrm{avg. empirically observed distance of a standard collision }} [/math] c) I do not know Edited February 18, 2012 by Xittenn
baxtrom Posted February 18, 2012 Posted February 18, 2012 So F is, who cares, and that is what I get from that! Since everbody's getting very excited about this, I'm going to try to explain why I posted that by now infamous 3rd reply to the topic. Yes, it was based on assumption, and I did believe it to be obvious but perhaps I should have spelled it out. The idea was, assuming the distance traveled by the COG of the car during the negative acceleration imposed by the contact condition could be approximated, a time average of the force acting on the car could be approximated (yes, that's that word again). I understand this might be a very upsetting thought and if so, please take a moment to catch your breath before you hit the "fast reply" button. Btw, I found this on a website hosted by Georgia State University's Department of Physics, with pictures and all. But hey, what were they thinking not doing a full dynamic elasto-plastic analysis using an explicit FEA solver to obtain the time history of all 12 stress and strain components in every inch of the car? Really, Georgia State.. -1
swansont Posted February 18, 2012 Posted February 18, 2012 There's an old saying: If we had some ice cream, we could have cake and ice cream, if only we had some cake. What we know about the problem is what was given in the OP: Hello Science Forums, I have a car travelling a certain velocity and absorbs x Joule of Kinetic energy on impact. I don't have the time duration of impact. I need to calculate the force at which the car made the impact. Can someone suggest me on how to find the force? We have the speed of the car and the amount of kinetic energy absorbed, but the actual number isn't given in the statement. We do not know the mass, we do not know the time and we do not know the distance. We don't even know if the KE increased or decreased. We can't apply conservation of momentum without knowing the time (and even that isn't enough). We can't apply work-energy without knowing the distance. There are too many unknowns to solve the problem. Period.
John Cuthber Posted February 18, 2012 Posted February 18, 2012 Doubt anything that blows your skirt up. But you are wrong. Force acting over time is directly relatable to momentum. Force acting over distance is directly relatable to time. When your question involves force and time the relevant quantity is momentum rather than enrgy. Again this is only true for a linear resistive load. It is in general false. OK, so among the things I doubt is that you actually considered the momentum of the system. It's easy enough to calculate the mass of the car from the velocity and the energy using 1/2 MV^2. But everyone agrees that there is not actually enough information to calculate the force asked for, even if you do calculate the momentum. So, the momentum does not actually help. If you still think I'm wrong then calculate them momentum and solve the problem. To a good approximation, at voltages high enough to kill, the body acts as a simple resistive load. It might not be true in general, but it's true in the particular instance that was defined.
Xittenn Posted February 18, 2012 Posted February 18, 2012 Since everbody's getting very excited about this, I'm going to try to explain why I posted that by now infamous 3rd reply to the topic. Yes, it was based on assumption, and I did believe it to be obvious but perhaps I should have spelled it out. The idea was, assuming the distance traveled by the COG of the car during the negative acceleration imposed by the contact condition could be approximated, a time average of the force acting on the car could be approximated (yes, that's that word again). I understand this might be a very upsetting thought and if so, please take a moment to catch your breath before you hit the "fast reply" button. Btw, I found this on a website hosted by Georgia State University's Department of Physics, with pictures and all. But hey, what were they thinking not doing a full dynamic elasto-plastic analysis using an explicit FEA solver to obtain the time history of all 12 stress and strain components in every inch of the car? Really, Georgia State.. There is no avg. subscript, there is no assertion of any kind that we can make any of the assumptions that you have proposed. In an academic sense the question is nilpotent. In an engineering sense, if an engineer were to do what you just proposed he would probably be fired. I know what it means to approximate, but the op did not assert the question well enough to come along and make more poor assumptions.
InigoMontoya Posted February 18, 2012 Posted February 18, 2012 In an engineering sense, if an engineer were to do what you just proposed he would probably be fired. As much as I hate to get involved in a flame war, I find that I have to disagree with this statement. It depends heavily upon your industry. In mine (ordnance), such approximations are made all the damned time: A customer comes in and wants something done. You ask them for more detailed information. They don't have it. You ask for the time/money to study the problem in more depth. They don't have that either. You find yourself in a position where you have a job to do, but not the resources to "do it right." Result: You make assumptions and engineering approximations not so dissimilar to the one being discussed... And if you do a halfway decent job of it, you're rewarded very well; not fired. 1
baxtrom Posted February 18, 2012 Posted February 18, 2012 As much as I hate to get involved in a flame war, I find that I have to disagree with this statement. In mine (ordnance), such approximations are made all the damned time: Result: You make assumptions and engineering approximations not so dissimilar to the one being discussed... And if you do a halfway decent job of it, you're rewarded very well; not fired. Thank you for adding some sense to the topic. I got some support from John too, appreciate that, and some critical but civil remarks from swansont, respect that. Had I known however that I would arouse the wrath of Dr Evil and his Bride I wouldnt have replied in the first case, however, alas, I naively tried to help with some humble ideas. Do I understand that the problem is not fully defined in the original post? Yes, I'm not an idiot in spite of mad drooling ramblings by Dr Evil above. The "car" could be a radio controlled toy or a 3 ton pickup, and the impact could involve another vehicle, a stone wall, an asteroid, a midget or anything in between. Right. So, those of you who are still steaming from indignation could use the "ignored users list" feature, it works beautifully, I tried. Ciao
swansont Posted February 18, 2012 Posted February 18, 2012 OK, so among the things I doubt is that you actually considered the momentum of the system. It's easy enough to calculate the mass of the car from the velocity and the energy using 1/2 MV^2. But you don't know the energy of the car. You know the change in kinetic energy of the car. As far as approximations go, yes, you could probably come up with an answer. If "somewhere between 1 and 10,000 N of force" (or whatever you get) is sufficient, that's fine. But we weren't asked for an estimate.
Xittenn Posted February 18, 2012 Posted February 18, 2012 As much as I hate to get involved in a flame war, I find that I have to disagree with this statement. It depends heavily upon your industry. In mine (ordnance), such approximations are made all the damned time: A customer comes in and wants something done. You ask them for more detailed information. They don't have it. You ask for the time/money to study the problem in more depth. They don't have that either. You find yourself in a position where you have a job to do, but not the resources to "do it right." Result: You make assumptions and engineering approximations not so dissimilar to the one being discussed... And if you do a halfway decent job of it, you're rewarded very well; not fired. You are right InigoMontoya I did make too broad a statement and it was inappropriate. In relation to automobile design and similar technologies I know for fact that this is an inappropriate statement and I say this because I have literally had to make parts with one ten-thousandth precision in the finished surface to accommodate the specs of the design. These industries do not nonchalantly inject weak statements into their work. Also, I am not flaming and I am disappointed in myself for not expressing myself clearly enough that someone would in fact see it otherwise! I am still trying to persuade my audience that the points made are in fact inappropriate.
John Cuthber Posted February 18, 2012 Posted February 18, 2012 There are worse assumptions than that the car comes to a halt. In the UK a collision sould have that effect, if only indirectly. It's true that the OP doesn't ask for an approximation, but it does ask for an answer. There are only really two options "there isn't enough information" or an approximation. There's nothing wrong with approximations and assumptions as long as they are recognised as such. In any event, I'm sure we are all looking forward to Dr Rocket solving the problem by the magic of momentum.
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