Very simple question regarding work and energy transfer

Drakkith

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.

DaleSpam

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

Not true. Many of the safety features in the car do not reduce the amount of energy transfered 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

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

thanks

selftaught

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

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.

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

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.

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.

DaleSpam

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.

Exactly, which is why you should try something else.

A falling body's energy is maximum at the top of the fall, energy has been transfered 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 transfered to the ground and the injury is maximum. Assuming a very soft surface a large amount of energy will be transfered 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.

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

selftaught

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.'

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

"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

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.


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.

Welcome to my world.

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

Rap

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

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.

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.

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

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.

That has been my position all along.

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.

DaleSpam

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

I mentioned fracture in Post 15.