Very simple question regarding work and energy transfer

[selftaught]

Hang on- not all energy can be used to perform work. Energy takes on many different forms, and can be interconverted between forms. The process is not 100% efficient (from the laws of thermodynamics), and so some energy is permanently 'lost', in the sense that it cannot be used to perform work.

I am focussed on mechanical energy primarily. In mechanics, the definition of energy is the capacity to do work. Given the conservation of energy, energy cannot be created nor destroyed. I would have thought 'permanetly lost' would equate to being destroyed. I understand the not 100% efficient idea, but the energy that is 'lost' transforms into another form of energy that then may be used to do work. If I'm incorrect I'd appreciate being corrected, but with a reconciliation with the idea that energy cannot be created nor destoryed and its definition as the capacity to do work.

Cheers :)

 

[Rap]

Thanks for the concern Drakkith. Yes I have understood everything that has been said. To a large degree, the responses have reflected my research. No agreement and no definitive explanation.

Ok, if you want to get serious, let's get serious. The best way to understand what is going on is to start with simple examples and build up to more complicated ones. A semi-elastic collision between two bodies is the most complicated, so that comes last.

First, you consider an elastic collision. Think of a small mass with a small spring to the right of it, moving with velocity V to the right. That's "the object". Then, to the right of it is a large mass with a large spring to the left of it. That's the Earth, its not moving. When the object and the Earth first touch, both springs start to compress. The Earth exerts a force on the object, compressing its spring and slowing it down, the object exerts an equal force on the Earth, compressing its spring and making the Earth move to the right. The Earth is so massive, that we can say it hardly moves at all. After a while, the object comes to rest because of the force of the Earth spring pushing back on it. Now both springs are in their maximum deflection. Some of the original kinetic energy is in the object spring, some of it in the Earth spring, the total of both is the original kinetic energy of the object when it first touched the Earth. The Earth and the object keep pushing on each other, and the object starts to move to the left, away from the Earth. Both springs start to decompress. When the object gets to the position where it first touched the Earth, both springs are back to their original unstressed length, and all of the energy that was contained in the springs has now been transferred to the kinetic energy of the object as it moves to the left, away from the Earth. To summarize, before the object touched the Earth, all the energy was its kinetic energy. During the collision, that kinetic energy was converted to the potential energy of the springs. Part of the kinetic energy went to potential energy of the Earth spring, part went to the kinetic energy of the object spring. Then, when the object started to move away from the Earth, the potential energy of both springs was transferred to the object, until it had regained all of the original energy which is now its kinetic energy, except it is moving to the left, not the right. As far as work is concerned, the total distance either object moved while the forces were in effect is zero, while the force at any point was the same going in as when going out. The work done on the object is zero - its energy is unchanged. The work done on the Earth is zero - it is still motionless.

Now let's do an inelastic collision. In this case, the entire kinetic energy of the colliding object gets converted to heat, and it sticks to the Earth, not by gravity, but because the collision is inelastic. We can think of an inelastic body ("object" or "Earth" or both) as a very damped spring, it gets compressed, but the energy that was formerly held by the compression of the spring is now converted into heat energy, and the spring stays compressed. The object has been deformed (or "injured"). Also, there is a partially elastic body, where its spring gets compressed and bounces back, but does not bounce back to its normal position. Part of the energy going into it is converted to heat, part is stored as potential energy of compression. There are many ways to have an inelastic collision. You can have the object be inelastic while the Earth is elastic or partially elastic, the object can be elastic or partially elastic, while the Earth is inelastic, or you can have both be inelastic. If both are partially elastic, the object will bounce off the Earth, but not as energetically as if the collision were elastic.

Lets say the object is inelastic, the Earth is elastic. Then, just as before, the object spring and the Earth spring get compressed until the object stops. The energy that would have been kept in the object spring as potential energy is now converted to heat, and the object spring stays compressed. The Earth spring expands, compressing the object spring even more, all of which gets converted to heat in the object. When the Earth spring has expanded to its normal position, the object and the Earth are motionless, and all of the original energy has been converted to heat contained in the object. The object spring is compressed (injured), and contains no potential energy. As far as work is concerned, the force on the object as it moves in is not the same as the force on the object as it moves out. Although the total distance moved while the force is in effect is zero, the forces are not the same at a particular point going in as going out. This means the integral of F dx is not zero, the work done on the object is not zero, but is equal to the original kinetic energy, all of which has been converted to heat.

Now let's say the object is elastic, the Earth is inelastic. The same thing happens, but in reverse. The object and Earth wind up motionless, but now all the kinetic energy of the object has been converted into heat that is contained in the Earth, and the Earth spring is permanently compressed, with no deformation or "injury" to the object. Work has been done on the Earth, not the object, and its energy has increased. All of the original kinetic energy of the object is now contained in the Earth as heat, and the Earth is deformed.

Notice that deformation is associated with conversion to heat. An elastic body does not deform and does not acquire heat in a collision.

Now we have to consider the real case. If the collision is not completely inelastic, the object will bounce off the Earth after the collision. In the real case, there is a little bounce, but let's say we can ignore that. Now we can say the collision is inelastic. But is the object inelastic while the Earth is elastic or partially elastic? Maybe the object is elastic or partially elastic while the Earth is inelastic? Maybe both are inelastic. All of these possibilities will yield an inelastic collision.

So now we have a more complicated set of possibilities. I think we can assume that the object is inelastic, since it gets very deformed. I'm not sure, but I think we might say that the Earth is inelastic as well. In this case, part of the initial kinetic energy goes into heating the object, part goes into heating the Earth. How much goes where depends on the strength of the object spring and the Earth spring. Since the Earth is much "harder" than the object, I think we can say that most of the kinetic energy is converted to heat in the object and the Earth ultimately absorbs only a small fraction of the energy. I might be wrong on this.

The process of the kinetic energy being absorbed by a real object is complicated. The simple idea of a spring does not cover it all. There are pieces sliding past each other, creating friction heat, there are damped springs being compressed, there is viscous effects of fluids being mechanically stirred which converts to heat. There is "flattening" of the object, but that probably doesn't account for a lot of the energy. There are sound waves generated in the object, which are eventually dissipated into heat. Also, there is a third object here - the atmosphere. Some of the energy gets converted to atmospheric sound which radiates away, but that probably doesn't account for a lot of the energy. The bottom line is that a good portion of the object's kinetic energy is passed to the Earth, which is then passed back to the object. Most of the kinetic energy winds up in the object as the form of heat, with the object being quite deformed.

If you go over the 23 points, I think that the above argument shows that:

5 is wrong, heat is generally associated with deformation, except for viscous stirring of fluids.

6 is wrong, there is no "energy of (permanent) deformation". Permanent deformation occurs because energy has been converted to heat.

7 is right, but incomplete - the strain energy is then released, to either impart kinetic energy, or converted to heat.

8 is correct but not covered by the above argument - This is a case where you could say the spring breaks. When this happens, pieces of the object are suddenly moved around, colliding elastically and semi-elastically with each other, creating heat and internal sound waves, all of which finally wind up as heat, with the resulting deformation being evidence of that process.

9+6 is incorrect. Again, and again, and again, the ground acquires energy from the object upon impact. You have to understand that this is true. Just because it seems hard does not mean it acquires no energy. If you have a very strong spring, it need hardly move at all in order to absorb a lot of energy.

9+7 - YES

9+9 wrong
9+10 wrong
9+12 Again, from the impacting object
9+13 Again, correct
9+14 Again, SE of the Earth is not zero, it acquires energy from the impacting object.

 

[Dale]

Most refer to heat and sound energy.

That is fine. I mentioned mechanical waves (sound) as a transient form of energy that quickly decays into heat. I prefer the term mechanical waves as it includes compression waves that are outside of the audio frequency as well as shear waves which are not generally associated with sound and probably cause the majority of the soft tissue damage in a fall. But as long as everyone understands that we are talking about a generalized "sound energy" then that is OK.

Strain energy refers to the ability of an object/body to restore itself to back to its original shape after being deformed. If the deformation goes beyond the limits of its ability to restore itself back to its original shape, eg. it is damaged, by definition it does not possesses strain energy. Hence, the KE was not transformed into strain energy.

Some of the body's KE does remain as elastic strain energy even after severe plastic deformations. Consider an undamaged body simply at rest on the ground compared to one in free fall. The falling body is unstrained and the body on the ground is elastically strained. Similarly for a damaged body at rest on the ground.

It's all helping in building an appreciation that this issue is not well considered nor understood.

If by not well understood you mean by you, then I agree. If you mean by the field in general, then you are simply wrong. The equations of motion have been around for about 3 centuries and are well known and validated. Additionally modern computational techniques make realistic modeling of viscoelastic materials quite feasible.

 

[Mapes]

Based on that logic, you'd be arguing that in a fall which results in damage and no deformation of the impact surface the kinetic energy accumulated during the fall is converted into heat and sound energy only. Therefore, injuries are caused by KE being converted into heat and sound energy. If that is your argument, then you're probably the first to articulate that argument. And I could not argue against you as I've not found anything that argues (or supports) that proposition.

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.

 

[selftaught]

Rap - thanks for the comprehensive response. With regards to the responses to my individual points:

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.

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.

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.

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.

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.

 

[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 the 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.