Very simple question regarding work and energy transfer

[selftaught]

Also note that when an object fractures, energy is stored via the creation of additional surface area. When a crystal permanently deforms, energy is stored via the creation of defects (dislocations) in the crystal. It's not all just heat.

And more concepts which do not reconcile with basic idea. Energy can only change form from one form of energy to another form of energy. What form of energy is 'additional surface area' or 'defects (dislocations)'? I'm not being smart but I'm simply sticking to one idea and following it through. Energy can only change form - so I only ask what form of energy.

 

[selftaught]

OK. A completely different response received from Ben Crowell (physics professor).

'In the example of the fracture, the energy is going into electrical potential energy to separate, e.g., one calcium atom from another.'

Just when I'm coming to grips with the conversion of KE to sound and heat energy when damage is inflicted on a body from a fall. ... Arghhhhhhhhh!

 

[Andy Resnick]

I am focussed on mechanical energy primarily.

Cheers :)

And what several people on this thread have been trying to tell you is that this is not sufficient to account for the phenomena you are discussing.

 

[Dale]

So after all of this there should be no doubt whatsoever that energy is definitely conserved. It has lots of different forms that it can change into. Is that clear to you now?

 

[Rap]

First of all, I thought about it, and there is the case, that I mentioned in a previous post, in which the Earth is totally rigid. I think I analyzed that wrong. You still have to think of the Earth as a spring. The force that a spring exerts is F=K D where K is the spring constant and D is the distance the spring moves from its no-force position. For a rigid spring, K is infinity and D is zero, but their product is not, its equal to the force on (or by) the spring. The potential energy contained in a compressed spring is K D^2/2. So for a rigid spring, since K D is a finite number this comes to zero. So that means a rigid object exerts a force without absorbing energy. The work done on the object is the integral of F dx where x is the distance that the object moves, not the Earth. So the true answer to your question "how can an object do work if it absorbs no energy" is not what I was saying i.e. that it cant, but that you don't need the Earth to absorb energy in order to exert a force. They are two different things. The energy that goes into deforming the object never leaves the object. The Earth does work on the object, not by absorbing and re-transmitting energy, but simply by exerting a force in this special case. The bottom line is that an infinitely massive, perfectly rigid body can exert a force without absorbing or transmitting energy.

1. '5 is wrong, heat is generally associated with deformation, except for viscous stirring of fluids.' Many texts use the example of a stone falling and on impact with the ground niether the stone nor the ground deforms and all the KE accumulated in the fall is transformed to sound and heat energy.

I think they are just skipping over the detailed analysis. The stone falls, the Earth exerts a force, which they ignore, then there is sound and heat.

2. Your # 6, 7, and 8 appear to be supporting my evolving assumption as to the transformation of KE when the tolerance levels of an object/body are exceeded, that is, it is converted into heat and sound energy only. If so, that is a simple answer to a simple question that has not been forthcoming from any of my research and I cannot find authoritative support for this proposition unfortunately. I wish I could because I could then move on.

Well, like I said, its more that tolerance levels for strain, where things start to break. There are a number of other processes going on which convert the kinetic energy to heat. The thing is, if the ground does not participate in the process, then the ground might as well not be there, and that would mean the object would pass through the Earth like it wasn't there. By participation, I mean the exertion of force on the object.

3. I'd love to gain support for your 9+6 as it would provide the answer I'm looking for, however, I cannot find support for it. As posted tonight, Benjamin Crowell's Conservation Laws provides examples where this is not the case. I'm still searching for authoritative support for this proposition.

Could you give a quote from Crowell where this is not the case? I would like to read it to see if I missed something or if you misinterpreted something. I think he may be making the assumption that the Earth is totally rigid.

4. Much of the rest of your responses are reflected in my #3 above. It's not that I'm arguing against you, it's that I cannot find authoritative support for the argument.

Your lengthy disertation, no offence intended, reflects what I've found in mechanics, physics, and biomechanics. Heaps of concepts, but they don't follow through with just one concept. For instance, when I research and discuss the work issue, most people cannot answer the question in energy terms and instead answer it in force terms. If injury is defined in energy terms, as it is in injury science, you want everything to be related to energy ... if you want it to be understood, respected, and used by non-academics. Also for instance, many suggest you can look at problems in terms of momentum and impulse, or, energy and work. I know that's true, but, if injury is defined in terms of energy 'exposure' (not transfer; see previous post) then you'd need to reconcile momentum with energy ... if its to be intelligible to those not versed in the many different concepts in these fields.

Conservation of momentum in this case is not very informative. Thats because the mass of the Earth is so huge. We violate the law of conservation of momentum when we say the object hits the Earth and stops, when actually the Earth moves towards the object a little. We should be able to talk entirely in terms of energy, and not worry about violating the conservation of momentum.

Just a simple thing like you're suggesting, energy is transferred from the impacting body to an apparently undeformed impact surface is then transferred to the impacting body resulting in deformation and work being performed by the impacting surface on the impacting body - after dozens and dozens of texts in mechanics, physics, and biomechnics, not one suggests this is the case. I'm not saying its not the case, I'm just saying they don't follow through with there explanations to explain this process.

It is a frustrating process (a) attempting to understand these simple questions, and (b) not finding any assistance in the numerous texts I've referred to.

With regard to "apparently undeformed surface" again, just because the Earth exerts a force does not mean it absorbs energy. In reality, it does, because there is no such thing as a perfectly rigid body. The work done on the object is the integral of the force by the Earth over the various deformations of the object.

I'm a physicist, and in everyday life, the conservation of momentum and energy are the pillars of every process. If I can't follow the energy and momentum and understand how it is conserved, then I don't understand the process. I have no experience in injury science, but I do in simple systems, like the one I outlined, where the flows of energy are clear-cut. I don't need references to prove what I am saying, I just need to be sure my thinking is straight. I understand that you need references to back things up and I understand the frustration of not finding them. Trust me on that, I been there and done that.

 

[Drakkith]

OK. A completely different response received from Ben Crowell (physics professor).

'In the example of the fracture, the energy is going into electrical potential energy to separate, e.g., one calcium atom from another.'

Just when I'm coming to grips with the conversion of KE to sound and heat energy when damage is inflicted on a body from a fall. ... Arghhhhhhhhh!

It takes energy to break bonds between atoms and molecules. This energy isn't always recoverable. It is conserved, but due to entropy you can't always get it back. Thats where a lot of your energy in your examples is going. To deform or break something you must expend energy to break molecular and atomic bonds. Think about your car example. It takes a LOT of energy (relative to the average human) to be able to destroy a car. Bending steel, aluminum, and other materials takes more energy than you as a person can put out.

As to what the kinetic energy is being converted into, you could say its get converted to potential and kinetic energy in the pieces of the car. (Things go flying, stuff bends, ETC) When you bend something, the energy is used to move the atoms and molecules to a different position, usually against the force keeping them together. When something breaks, the objects can snap back if it is "stiff" enough, releasing the potential energy that was used as vibrations, heat, ETC. (Minus the energy that was used to BREAK the bonds in the material) If an object is not stiff at all and easily bendable, then when you deform it it contains no potential energy. The energy has been used, and is still conserved, but I don't think it is able to be recovered. (Not 100% sure all this is 100% correct, but I hope you get what I'm saying)

 

[selftaught]

So after all of this there should be no doubt whatsoever that energy is definitely conserved. It has lots of different forms that it can change into. Is that clear to you now?

That has never been in question. To make it clear - I am interested in explaining the injury or damage that results when a person lands from a fall onto a hard surface. And that explanation has to be in terms of energy. GPE is converted to KE which is then converted to what form of energy.

Absorb, dissipate, deform, release are not forms of energy. These are concepts which have been used to describe what is happening, but, it does not answer the simple question: KE of a fall is converted into what form of energy? Simple enough question I would have thought, but one which attracts some many other concepts other than forms of energy.

Deform is not a form of energy. Deformation energy is, when the material has the ability to restore itself to its original shape. A person is not injured if the KE is converted to deformation energy, aka strain energy. When the deformation goes beyond the materials ability to restore itself, the KE has be converted into some other form of energy.

Sound and heat energy have been referred to in texts, though not in connection with then being the final energy conversion in this process. FINALLY, one person posting started saying the same thing, albeit I'd suggest being directed by my questioning their responses. Then Crowell throws in electrical energy seperating atoms in the case of fracture into the mix. Something that nobody has considered in this thread to date.

Not wishing to be rude, but is the unclear, undefinitive nature of this issue clear to you now.

 

[Drakkith]

From Wikipedia:

In the context of physical sciences, several forms of energy have been defined. These include:

Thermal energy, thermal energy in transit is called heat
Chemical energy
Electrical energy
Radiant energy, the energy of electromagnetic radiation
Nuclear energy
Magnetic energy
Elastic energy
Sound energy
Mechanical energy
Luminous energy

So that's the short answer. Pick any of these and I can almost gurantee you that some of the energy will be transformed into it. (Other than maybe Nuclear Energy) I'm not seeing anything undefinitive with any of our answers. The only problem I'm seeing is that you don't know any basics on the different forms of energy. So when someone says "The energy is used in deforming the object", you don't understand what that means. And then because you don't understand it, you claim it to be unclear or undefinitive when it is definitively not.

Absorb, dissipate, deform, release are not forms of energy. These are concepts which have been used to describe what is happening, but, it does not answer the simple question: KE of a fall is converted into what form of energy? Simple enough question I would have thought, but one which attracts some many other concepts other than forms of energy.

While you may be correct here, you have to realize that it's not always a simple answer to what you think is a simple question. How can one explain potential energy if you don't understand how it is converted and how you get it in the first place? Everything is interconnected and related, and to get a grasp on this you need to understand a few basics first. Trust me, once you understand some of the basics, a lot of the answers simply fall into place. =)

When the deformation goes beyond the materials ability to restore itself, the KE has be converted into some other form of energy.

From wikipedia again:

Elastic energy of or within a substance is static energy of configuration. It corresponds to energy stored principally by changing the inter-atomic distances between nuclei. Thermal energy is the randomized distribution of kinetic energy within the material, resulting in statistical fluctuations of the material about the equilibrium configuration. There is some interaction, however. For example, for some solid objects, twisting, bending, and other distortions may generate thermal energy, causing the material's temperature to rise. Thermal energy in solids is often carried by internal elastic waves, called phonons. Elastic waves that are large on the scale of an isolated object usually produce macroscopic vibrations sufficiently lacking in randomization that their oscillations are merely the repetitive exchange between (elastic) potential energy within the object and the kinetic energy of motion of the object as a whole.

When a car, or a person, is dropped, the energy is kinetic energy is converted into heat, vibrations, further elastic waves in the material, and so forth. If the energy is too great, then materials start to break and deform. The extra energy after something deforms is STILL is converted to the above though. It can result in further deformation, or simply vibration waves or heat. Call it potential energy, or chemical energy, or whatever, but it is most definitively converted to something specific. This is just a really complicated situation and I'm not very good at explaining things in 100% correct specific terms.

 

[Dale]

I am interested in explaining the injury or damage that results when a person lands from a fall onto a hard surface. And that explanation has to be in terms of energy.

Why? It seems rather counterproductive to artificially restrict the domain of possible answers this way.

If someone asked you to explain why 2+2=4 and stated that the explanation has to be in terms of division it would probably be possible to make the explanation, but it would be unnecessarily cumbersome and confusing. Not because either addition or division is poorly understood, but just because you are asking for a bad conceptual framework to answer the question.

With specific regards to your question here, it does in fact take energy to cause injury, and energy is conserved, however, conservation of energy is not a particularly useful principle for quantifying injury. For example, the energy transferred to a body by falling 10 m is equal to the energy transferred by raising the body temperature about 0.02º C, and yet the injury is far different. So why artificially choose energy as your key concept for explaining injury if the amount of energy is so completely unrelated to the severity of injury?

 

[Mapes]

Then Crowell throws in electrical energy seperating atoms in the case of fracture into the mix. Something that nobody has considered in this thread to date.

Not true; look at my post #34. The creation of new surface area (which occurs in fracture) stores energy because (electrostatic) atomic bonds have been broken.

And more concepts which do not reconcile with basic idea. Energy can only change form from one form of energy to another form of energy. What form of energy is 'additional surface area' or 'defects (dislocations)'? I'm not being smart but I'm simply sticking to one idea and following it through. Energy can only change form - so I only ask what form of energy.

Broken atomic bonds. (This applies to both additional surface area and crystal defects.)

You seem to be aggravated that it's taken a few pages of posts to figure out precisely what you're looking for. Your original question was "I can understand work in this situation in terms of W=Fd and Newton's law of action-reaction. I cannot understand work in terms of energy transfer in this situation as the object or ground possessed no energy to begin with." It was not "When someone falls and is motionless and injured, please delineate exactly how their original kinetic energy is now apportioned."

 

[selftaught]

I want to respond to the new posts but 'mulit quote' does not appear to be functioning for me for some reason. I'm new to using this (any) forum. Can someone help please?

 

[Dale]

Click "multi quote" on each of the different posts that you want to quote, then click "quote" on anyone of them to actually start.

 

[selftaught]

Click "multi quote" on each of the different posts that you want to quote, then click "quote" on anyone of them to actually start.

Thanks DaleSpam. Will follow your instructions now. I must confess I did attempt that but was concerned that clicking quote would include my comments in with the actual quote ... as I did in reply to one poor sod's post when he was attempting to help me.

 

[selftaught]

OK. For one and all. Before things get out of hand, I need to explain where I'm coming from. Firstly, I truly appreciate your attempts at helping me. Secondly, as my name suggests, I am in the process of teaching myself certain mechanical/physics principles. I am doing so for a very specific purpose. I am attempting to use these principles to facilitate the understanding and study the techniques of combatives (or whatever name is applied to this activity). I was originally attempting to put a 'little science' behind the how-to instruction of say striking techniques. I saw others attempt to do the same. They referred to momentum, kinetic energy, force, elastic and inelestic collissions, impulse, and work. After I'd read their explanation of these concepts, I was left with one question - 'so what?'. It didn't assist in my understanding and study of these techniques at all. So I approached the problem from the opposite direction and asked, 'what causes an injury?'. Here I discovered a relatively new science (1961) that defined injury in terms of 'exposure' to energy. Voila. Perfect, one concept that can be used to explain injury. So any other concept I want to relate back to energy, otherwise, I'm in the same situation of using numerous concepts to explain things which the reader cannot link back to injury.
When defining injury in terms of energy, some refer to transfer instead of exposure. As I posted previously, some like the World Health Organisation differentiate between transfer and what they refer to as the prohibition of the transfer of energy. The transfer of energy from a punch say, is easy peasy. However, how the kinetic energy accumulated in a fall causes an injury has proven illusive. My problem was narrowed down to 2 questions. 1. Based on the conservation of energy and that energy can only change form, and that a person's body is deformed upon impact from a fall, I wanted to know specifically what form of energy the KE converted to upon impact and when damage occurs. This has largely been answered and given me a direction to research. Thank you. 2. How does the concept of work explain the damage sustained when landing from a fall? This is still a little confusing which will be evident in some of the responses I'll give to some of the posts.
Thank you all for helping. What I'm trying to do is use theory to help practice, a job that is very poorly done in my experience (not just in physics and biomechanics, but in most disciplines). I think one of the problems may be that there has been a degree of miscommunication because you speak technese and I don't to a large degree. A common fault when specialists attempt to talk to those outside their tribe. The truly remarkable are those who are bilingual in this regard.

 

[selftaught]

Why? It seems rather counterproductive to artificially restrict the domain of possible answers this way.

If someone asked you to explain why 2+2=4 and stated that the explanation has to be in terms of division it would probably be possible to make the explanation, but it would be unnecessarily cumbersome and confusing. Not because either addition or division is poorly understood, but just because you are asking for a bad conceptual framework to answer the question.

With specific regards to your question here, it does in fact take energy to cause injury, and energy is conserved, however, conservation of energy is not a particularly useful principle for quantifying injury. For example, the energy transferred to a body by falling 10 m is equal to the energy transferred by raising the body temperature about 0.02º C, and yet the injury is far different. So why artificially choose energy as your key concept for explaining injury if the amount of energy is so completely unrelated to the severity of injury?

DaleSpam. Tried your advice but only got this one long quote. Ok for this reply but not for others. My previous post today explains why I am restricting my domain. I'm trying to apply mechanical principles to explain a particular activity. That restriction is based on the definition of injury as being exposure to energy, and in the case of an injury resulting from landing from a fall, mechanical energy. There is no 'artificiality' about choosing energy as the key concept in explaining injury. There is a whole science developed around the concept that injury is caused by energy. All the safety features in your car have arisen from this science. And this science has only been in existence since William Haddon in 1961. Quite remarkable really. It is a unique example of science actually and directly, and designed to be, to improve the quality of life, if not life itself.

 

[selftaught]

Not true; look at my post #34. The creation of new surface area (which occurs in fracture) stores energy because (electrostatic) atomic bonds have been broken.

Broken atomic bonds. (This applies to both additional surface area and crystal defects.)

You seem to be aggravated that it's taken a few pages of posts to figure out precisely what you're looking for. Your original question was "I can understand work in this situation in terms of W=Fd and Newton's law of action-reaction. I cannot understand work in terms of energy transfer in this situation as the object or ground possessed no energy to begin with." It was not "When someone falls and is motionless and injured, please delineate exactly how their original kinetic energy is now apportioned."

Again the multiquote didn't work. Too true, Mapes. You did refer to 'new surface area' in post 34. The first to suggest more than sound and noise energy. However, as explained in a previous post today, I'm not from your tribe and that phrase would not suggest a form of energy to anyone outside your tribe. My understanding of the forms of energy are as listed in a post by someone else reference Wikipedia, although my understanding came from more reliable sources.

Your right. I started out asking the question about the relationship between work and injury from a fall. I was then bombarded with a multitude of concepts other than work and energy so I introduced my second question which I advised I was saving for an additional post. This may have confused the issue, for which I apologise. I've got something to work with now concerning what form of energy the KE accumulated in a fall is converted to upon impact. It took a while, and is not easily sourced in any of the dozens of texts I've read on physics, mechanics, and biomechanics, but I've got more than most in explaining this issue. The influence of work on injury, with work being defined as, among other things, the means by which energy is transferred from one object or system to another is still unclear - particularly given Crowell's latest brief email to me on the subject.

Cheers

 

[Drakkith]

2. How does the concept of work explain the damage sustained when landing from a fall?

Lets do something simple first. If I drop a bowl on the ground and it shatters, how does work explain that?


Edit: Yes, I am asking you. This is a 2 way conversation and I don't know what you know compared to me. It is entirely possible you or I have the wrong idea about work and force and energy, and the only way to tell is to talk!

 

[selftaught]

For me pressing muti quote then quote, or pressing quote, are producing the same result. One long quote of the entire post rather than each paragraphe being a quote which i can then comment on. I'm obviously doing something wrong. Any suggestions?

 

[Drakkith]

Press the multi quote for each post you want to quote, then hit New Reply, not quote.

 

[Dale]

DaleSpam. Tried your advice but only got this one long quote. Ok for this reply but not for others.

Multi quote allows you to respond to several different posts at once. There is no automatic functionality for separately quoting individual paragraphs. What I do is I quote a post, then I copy the open-quote to the end so that I have a close quote followed by an open quote. Then I copy and paste that at each spot where I would like to insert my reply.

That restriction is based on the definition of injury as being exposure to energy

That is a bad definition. Mere exposure to energy does not cause injury, as I illustrated above.

There is a whole science developed around the concept that injury is caused by energy. All the safety features in your car have arisen from this science.

Not true. Many of the safety features in the car do not reduce the amount of energy transferred to the passengers. In fact, about the only one that I can think of which does is anti-lock brakes which reduce the speed of the vehicle prior to the collision. Safety belts, air bags, and crumple zones all reduce the force and acceleration sustained, but not the amount of energy transfered. Reducing the force, in turn, reduces the strain, which is the final cause of mechanical injury.

 

[Rap]

I think everything can be explained in terms of energy.

To simplify things, potential energy converts to kinetic energy, that's easy. When the object hits the earth, almost all of the kinetic energy ultimately gets converted to heat, because the object is no longer moving. That's easy too. The question of what are steps by which the kinetic energy gets converted to heat, that's the complicated part. Following where the energy goes from the time the object first touches the earth, until the collision is complete, is complicated.

Lets ignore all the small contributions:

1) Energy which deforms the Earth - If the Earth were elastic, but not rigid, it would absorb some of the energy and give it back to the object. If the Earth were elastic and perfectly rigid, it would absorb no energy, but it still would exert a force on the object.

2) Sound energy - A small amount of energy is lost in sound waves, which ultimately dissipate in the atmosphere as heat.

I think this about covers the energy lost by the object to other systems. That means most of the kinetic energy conversions to heat occur in the object as it collides with the Earth. Here are my ideas about the mechanisms by which the kinetic energy is converted to heat in order of importance, assuming the object is a human body. Please add to the list, I'm sure I am forgetting some.

1) Broken atomic bonds - tearing and breaking of things. Think of this as a spring that you stretch until it snaps. You add energy to it, it holds it as potential energy, the spring snaps, it vibrates but the vibration is damped, and all that potential energy goes to heat. During the collision, the energy added is due to the force of the Earth on the contact part of the object, then force transmitted to the part above it, etc.

2) Internal pressure waves or sound waves (compression and shear) which dissipate to heat.

3) Inelastic rearrangement of parts. This is what I was modeling as an inelastic spring, one that compresses, but does not bounce back. The permanent compression holds no potential energy - the energy that went into the compression was not used to bounce the spring back, but was rather dissipated as heat.

4) Permanent elastic deformation - Parts of the body may act as springs which are compressed, do not lose their stored potential energy as heat, but are prevented somehow from springing back. Like a bone that gets bent and lodged between two other bones or something. This is energy that is not converted to heat. I think this contribution is quite small.

 

[selftaught]

Lets do something simple first. If I drop a bowl on the ground and it shatters, how does work explain that?


Edit: Yes, I am asking you. This is a 2 way conversation and I don't know what you know compared to me. It is entirely possible you or I have the wrong idea about work and force and energy, and the only way to tell is to talk!

OK. Thanks. I see it can be explained two different ways which I cannot reconcile.

1. When work is described as the means by which energy is transferred from one object or system to another. And W=Fd. The bowl accumulates KE during its fall. The ground possesses no mechanical energy: KE+GPE+SE=0. Thus, the bowl has the capacity to do work on the ground but the ground does not have the capacity to do work on the bowl, unless, the work done by the bowl on the ground transfers its KE to the ground in the form of SE which is then transferred back to the bowl resulting in damage. (I'm still working on RAP's expanation). However, there is conflicting opinion as to whether or not the ground does work on bowl. Some suggest the work is internal to the bowl. Bottom line, I'm looking to understand the application of work in this context in terms of the transfer of energy only.

2. From a force perspective, easy peasy. W=Fd and Newton's third law. Bowl applies force to floor which applies reaction force in turn. Bowl deforms so work is done on bowl by floor. This is where it gets a bit tricky. The floor does not deform so no work is done on the floor. However, Crowell explained that if work is done on one object, work is also done on another. If no displacement of the floor ...? See above as regards to infintessimal deformation and transfer.

I keep on getting caught up with the transfer of energy, and this is the concept I am interested in.

Thanks

 

[selftaught]

Press the multi quote for each post you want to quote, then hit New Reply, not quote.

thanks

 

[selftaught]

Multi quote allows you to respond to several different posts at once. There is no automatic functionality for separately quoting individual paragraphs. What I do is I quote a post, then I copy the open-quote to the end so that I have a close quote followed by an open quote. Then I copy and paste that at each spot where I would like to insert my reply.

THanks. Giving it a try. Found I also had to copy the quote to the front of subsequent paragraphs.

That is a bad definition. Mere exposure to energy does not cause injury, as I illustrated above.

You'd be arguing against an entire scientific discipline. They refer to five forms of energy: mechanical, chemical, electrical, radiant, and thermal. There is also the absence of vital elements such as oxygen or heat but the main thrust is exposure to energy. The reference to 'exposure' proves problematic, as you can see, when explored.

Not true. Many of the safety features in the car do not reduce the amount of energy transferred to the passengers. In fact, about the only one that I can think of which does is anti-lock brakes which reduce the speed of the vehicle prior to the collision. Safety belts, air bags, and crumple zones all reduce the force and acceleration sustained, but not the amount of energy transfered.

Again, you'd be arguing against an entire scientific field. They talk about amounts and at rates. And in this one paragraph you've uncovered my dilemma. 'Transferred' - explain that in terms of energy and landing from a fall. Force is involved in impact, and KE is pre-impact.

If you're interested in the application of mechanics to study injury and teh causes of injury, search for William Haddon, who is the so-called father of this field. Once he conceptualised injury in terms of 'energy exchange' he was appointed to a new office in the US government responsible for road safety.

Your last paragraph encaptulates my problem. KE is pre-impact, force is impact, in the three phases of an injury event. Given the definition of injury in terms of energy exposure, exchange, or transfer, I'm then attempting to reconcile any impact explanation with the initional pre-impact injury causeing energy.

 

[Rap]

OK. Thanks. I see it can be explained two different ways which I cannot reconcile.

1. When work is described as the means by which energy is transferred from one object or system to another. And W=Fd. The bowl accumulates KE during its fall. The ground possesses no mechanical energy: KE+GPE+SE=0. Thus, the bowl has the capacity to do work on the ground but the ground does not have the capacity to do work on the bowl, unless, the work done by the bowl on the ground transfers its KE to the ground in the form of SE which is then transferred back to the bowl resulting in damage. (I'm still working on RAP's expanation). However, there is conflicting opinion as to whether or not the ground does work on bowl. Some suggest the work is internal to the bowl. Bottom line, I'm looking to understand the application of work in this context in terms of the transfer of energy only.

I modified myself on that (see my post #40) - In the special case of an infinitely massive, perfectly rigid (hard) Earth, the Earth has the capacity to do work on the bowl without the Earth deforming. In this case, the Earth exerts a force on the bowl, but absorbs no energy. If the bowl emits no sound as it breaks, and after the collision, the bowl pieces are motionless, then you know, by conservation of energy, that the work done by the Earth on the bowl is equal to the kinetic energy of the bowl when it first touches the Earth, and all of that energy has been converted to heat, because there is no longer any kinetic energy. The work that was done is the result of the force by the Earth acting on the bowl.

Now comes the hard part - What are the processes by which the bowl's kinetic energy gets converted to heat? This is what we should concentrate on.

2. From a force perspective, easy peasy. W=Fd and Newton's third law. Bowl applies force to floor which applies reaction force in turn. Bowl deforms so work is done on bowl by floor. This is where it gets a bit tricky. The floor does not deform so no work is done on the floor. However, Crowell explained that if work is done on one object, work is also done on another. If no displacement of the floor ...? See above as regards to infintessimal deformation and transfer.
Thanks

This is correct, and equivalent to what I said above - it assumes an infinitely massive, perfectly rigid Earth. It is correct up to the point where you quote Crowell. Crowell says that if work is done on one object, it is done on the other. This means he is NOT assuming a perfectly rigid Earth. Since there is no such thing as a perfectly rigid body he is correct, but if there were, he would be incorrect. Nevertheless, we can make the approximation that the Earth is perfectly rigid. If it is close to rigid, we will get results that are close to the truth.

The confusion results from not knowing whether a perfectly rigid Earth is being assumed or not.

 

[Dale]

You'd be arguing against an entire scientific discipline.

It is more likely that I am arguing against your misunderstanding of an entire scientific discipline. But if you are correctly understanding the discipline then the discipline is in error. It will not be the first time.

The reference to 'exposure' proves problematic, as you can see, when explored.

Exactly, which is why you should try something else.

And in this one paragraph you've uncovered my dilemma. 'Transferred' - explain that in terms of energy and landing from a fall. Force is involved in impact, and KE is pre-impact.

A falling body's energy is maximum at the top of the fall, energy has been transferred to it by the elevator. During the fall there is little or no energy transfer at all. Assuming a rigid surface almost no energy is transferred to the ground and the injury is maximum. Assuming a very soft surface a large amount of energy will be transferred to the ground and the injury will be a minimum. In either case the ground does not transfer energy to nor do work on the body, the body transfers energy to and does work on the ground, resulting in a reduction of injury. The energy transfer theory is not very successful here.

Given the definition of injury in terms of energy exposure, exchange, or transfer, I'm then attempting to reconcile any impact explanation with the initional pre-impact injury causeing energy.

And you are having difficulty in that reconciliation because the definition is fundamentally bad.

 

[selftaught]

I modified myself on that - In the special case of an infinitely massive, perfectly rigid (hard) Earth, the Earth has the capacity to do work on the bowl without the Earth deforming. In this case, the Earth exerts a force on the bowl, but absorbs no energy. If the bowl emits no sound as it breaks, and after the collision, the bowl pieces are motionless, then you know, by conservation of energy, that the work done by the Earth on the bowl is equal to the kinetic energy of the bowl when it first touches the Earth, and all of that energy has been converted to heat, because there is no longer any kinetic energy. The work that was done is the result of the force by the Earth acting on the bowl.

Specific questions.

1. 'The Earth has the capacity to do work'. Energy is defined as the capacity to do work. What form of energy does the Earth have which then has the capacity to do work?

2. I understand the idea that the reaction force applied by the Earth causes displacement in terms of deformation of the bowl. The recently introduced idea that work done on one object is mirrored by work done on the other object causes me a problem. The bowl applies a force on the earth, but there is no displacement. No displacement, no work according to the most basic of texts. ... Or is that the answer. The displacement of the Earth as a result of the work done on it by the bowl is so infintessimal as to approximate zero displacement.

... bugger, if so, I still come back to, if the Earth does work on the bowl it transfers energy. What energy?

Crowell just threw a spanner in the works in emailing me now that 'work is done on the ground.'

Now comes the hard part - What are the processes by which the bowl's kinetic energy gets converted to heat? This is what we should concentrate on.

No, no, no. That's not what I'm interested in. It may be of generic interest but I'm focussed on energy and the transfer of energy to cause damage only .

 

[Rap]

A falling body's energy is maximum at the top of the fall, energy has been transferred to it by the elevator. During the fall there is little or no energy transfer at all. Assuming a rigid surface almost no energy is transferred to the ground and the injury is maximum. Assuming a very soft surface a large amount of energy will be transferred to the ground and the injury will be a minimum. In either case the ground does not transfer energy to nor do work on the body, the body transfers energy to and does work on the ground, resulting in a reduction of injury. The energy transfer theory is not very successful here.

"In either case..." - this is not correct. On a soft surface the object pushes into the soft surface which absorbs energy, then rebounds, transferring energy back to the object. In either case, the ground DOES do work on the object, or else its energy would not change. Finally, assuming a rigid surface, the body transfers no energy to the ground, and does no work on it. It exerts a force on the rigid surface, but since the rigid surface does not move, the dx in the equation W=F dx is zero and the work is zero. Looking at it another way, the rigid surface absorbs no energy, and so no work has been done on it.

 

[selftaught]

It is more likely that I am arguing against your misunderstanding of an entire scientific discipline. But if you are correctly understanding the discipline then the discipline is in error. It will not be the first time.

Not a lot of room to misunderstand. One of the fundamental breakthroughs of injury science was James Gibsons identification of energy as being the 'agent of injury' in 1961. William Haddon can to the same conclusion independently 3 months later and clarified some confusion surrounding Gibson's explanation by including a vehicle of vector that conveyed the agent of injury, energy.

Exactly, which is why you should try something else.

Can't. Given the definition of injury. If there was a definition that said injury was caused by the application of momentum or something else, all good, but to structure an argument I need support.

A falling body's energy is maximum at the top of the fall, energy has been transferred to it by the elevator. During the fall there is little or no energy transfer at all. Assuming a rigid surface almost no energy is transferred to the ground and the injury is maximum. Assuming a very soft surface a large amount of energy will be transferred to the ground and the injury will be a minimum. In either case the ground does not transfer energy to nor do work on the body, the body transfers energy to and does work on the ground, resulting in a reduction of injury. The energy transfer theory is not very successful here.

Welcome to my world.

BTW, thanks for taking the time to help me with this problem.

 

[Rap]

1. 'The Earth has the capacity to do work'. Energy is defined as the capacity to do work. What form of energy does the Earth have which then has the capacity to do work?

The quick answer is that energy is NOT defined as the capacity to do work.

2. I understand the idea that the reaction force applied by the Earth causes displacement in terms of deformation of the bowl. The recently introduced idea that work done on one object is mirrored by work done on the other object causes me a problem. The bowl applies a force on the earth, but there is no displacement. No displacement, no work according to the most basic of texts. ... Or is that the answer. The displacement of the Earth as a result of the work done on it by the bowl is so infintessimal as to approximate zero displacement.

That is the answer. The recently introduced idea does not assume a rigid Earth (or floor). If the Earth is perfectly rigid, then no work is done on the Earth.

... bugger, if so, I still come back to, if the Earth does work on the bowl it transfers energy. What energy?

For a rigid Earth, it does not transfer energy, it exerts a force. This force acts on the body causing it to convert its kinetic energy to heat energy, but it does not transfer any energy. The rigid Earth absorbs no energy, and has no energy of its own, so there is none to transmit.

Crowell just threw a spanner in the works in emailing me now that 'work is done on the ground.'

No spanner, he is simply saying that the Earth is not perfectly rigid and he is right, it is not. There is no such thing as a perfectly rigid body. That does not mean that we cannot pretend it is rigid. If it is close to rigid, we will get results that are close to the truth. Send him your quote and this response, I believe he will agree.

 

[selftaught]

The quick answer is that energy is NOT defined as the capacity to do work.

Quite honestly, I don't know where to go from there. I can quote numerous texts which describe energy in exactly these terms. McGinnis, Biomechanics of Sport and Exercise, 'in mechanics,energy is defined as the capacity to do work' (p105). The same in Hall, Basic Biomechanics, p 408. Carr, Sport Mechanics for Coaches, p44. Whiting and Zernicke, Biomechanics of Musculoskeletal Injury, p56. I quote these because these are the texts open on my desk for the past week or two while I've tried to come to grips with this issue. Then there are the large number of other texts that I've referred to that define likewise.

The rigid Earth absorbs no energy, and has no energy of its own, so there is none to transmit.

That has been my position all along.

No spanner, he is simply saying that the Earth is not perfectly rigid and he is right, it is not. There is no such thing as a perfectly rigid body. That does not mean that we cannot pretend it is rigid. If it is close to rigid, we will get results that are close to the truth. Send him your quote and this response, I believe he will agree.

My apoligies. My cut and paste was shoddy. What Crowell said was 'No work is done on the ground' in the case of a body impacting with the ground. No work done on the ground by the body means, according to Crowell and agreed to here by I can't remember who, no work done on the body by the ground. Thus work does not get done.

In this case, what I think I'm left with is that upon impact no work is done, therefore no energy is transferred. The damage is explained in that the KE is not transferred but retained in teh body and converted into sound, heat, electrical, etc energy resulting in deformation, permanent deformation, and/or damage. A force has been applied by the body and the ground has applied a reaction force, and since there was no change in energy in the ground no work has been done.

Now, if this is correct, all I've got to find is support for this argument as there are a number of texts which refer to the damage to body and car from an impact with a solid object or Earth as being the cause of work done on it. Unless, ... I read one text which explained that 'scientists' used the terms transfer and transform to refer to the same thing. In this case, if transfer is used to refer to transfer from one form to another, the 'work done' is the transfer of KE to the other form of energy in the body. Of course I run into the problem of identifying the agents of force and reaction force in this case, given work is done on both agents.

... my head is hurting. I'm done for a while. Cheers. Thanks for the help.

Instead, the KE is converted to sound, heat, and electrical energy (ignoring for the moment such specialised concepts as crystals and large area or whatever) the results in deformation, permanent deformation, and damage in the fallen body.

 

[Dale]

Given the definition of injury.

The definition is clearly wrong.

I don't know James Gibson's background, but William Haddon was a MD, not a physicist. A MD's training is focused on medicine and not physics, and thus as a group their grasp of physics is loose at best. If you want to use that kind of loose interpretation of physics then don't expect to be able to do the kind of rigorous analysis that you seem to want, it simply won't work.

 

[Andy Resnick]

OK. A completely different response received from Ben Crowell (physics professor).

'In the example of the fracture, the energy is going into electrical potential energy to separate, e.g., one calcium atom from another.'

Just when I'm coming to grips with the conversion of KE to sound and heat energy when damage is inflicted on a body from a fall. ... Arghhhhhhhhh!

I mentioned fracture in Post 15.

 

[Rap]

Quite honestly, I don't know where to go from there. I can quote numerous texts which describe energy in exactly these terms. McGinnis, Biomechanics of Sport and Exercise, 'in mechanics,energy is defined as the capacity to do work' (p105). The same in Hall, Basic Biomechanics, p 408. Carr, Sport Mechanics for Coaches, p44. Whiting and Zernicke, Biomechanics of Musculoskeletal Injury, p56. I quote these because these are the texts open on my desk for the past week or two while I've tried to come to grips with this issue. Then there are the large number of other texts that I've referred to that define likewise.

Ah - it says "in mechanics, energy is defined as the capacity to do work". This is not generally true. In thermodynamics, energy cannot be totally converted into work, so this definition fails. And we are doing more than mechanics here, we are doing thermodynamics, because we are dealing with heat. Maybe it would be better to think of kinetic energy or potential energy as the capacity to do work, but not heat energy.

My apoligies. My cut and paste was shoddy. What Crowell said was 'No work is done on the ground' in the case of a body impacting with the ground. No work done on the ground by the body means, according to Crowell and agreed to here by I can't remember who, no work done on the body by the ground. Thus work does not get done.

Yes, that is true, but in the rigid Earth case, the body does work on itself. Please, let's consider a simple problem, where we know exactly what is going on, and watch what happens to the energy. If you don't understand this simple problem, then let me know, don't just skip over it or ignore it. Take a mass connected to a spring to its right, moving to the right, towards a rigid Earth. Let's say the spring can be compressed, but it will not expand, so that you can add energy of compression to it, but you cannot extract it. Some kind of racheting mechanism or something. When the spring touches the Earth, it exerts a force on the Earth, and the Earth exerts an equal force back. The spring compresses. The Earth is doing no work on the spring, because, on its end of the spring, there is no motion. The spring is not doing any work on the Earth, again, because there is no motion there. The amount of work done is dW=F dx and dx is zero. Work IS being done on the spring, by the moving mass however. It is exerting a force on the spring, equal and opposite to the force the spring exerts on it. Here, there is motion, dx is not zero, the kinetic energy of the mass is being transferred to the spring. When the spring reaches its maximum compression, the velocity of the mass is zero. But there is no rebound, because the spring cannot expand. So now the total original kinetic energy of the mass is bound up in the potential energy of compression of the spring, and nothing is moving. (Please note that I was wrong when I said the rigid Earth did work on the object)

Now you can extend this to the complicated case - For a rigid Earth, no work is done by the ground on the object, no work is done by the object on the ground, but different parts of the body perform work on different other parts, the net result of which is to convert almost all of the original kinetic energy of the body into heat.

In this case, what I think I'm left with is that upon impact no work is done, therefore no energy is transferred. The damage is explained in that the KE is not transferred but retained in teh body and converted into sound, heat, electrical, etc energy resulting in deformation, permanent deformation, and/or damage. A force has been applied by the body and the ground has applied a reaction force, and since there was no change in energy in the ground no work has been done.

Yes.

Now, if this is correct, all I've got to find is support for this argument as there are a number of texts which refer to the damage to body and car from an impact with a solid object or Earth as being the cause of work done on it. Unless, ... I read one text which explained that 'scientists' used the terms transfer and transform to refer to the same thing. In this case, if transfer is used to refer to transfer from one form to another, the 'work done' is the transfer of KE to the other form of energy in the body. Of course I run into the problem of identifying the agents of force and reaction force in this case, given work is done on both agents.

I think that from looking at the above example, you can see that there are two systems, the mass and the spring, and the kinetic energy of the mass is transferred to the potential energy of the spring. This could also be thought of as a transformation of the mass-spring system.

 

[Drakkith]

Perhaps we are looking at this the wrong way. Think of the body, or any object really, as not 1 object, but LOTS. Think of it as it's composite particles. In this case it is easy to see how an arm can exert work on the bone causing it to fracture. If you look at each individual spot in the body, you can see where they are getting their energy from, what is doing work on them, and the result of that work. Add it all up into a macroscopic effect and you get injuries.