Want to understand ke formula without having to accept any assumptions

[speedingelf]

I explained earlier that I saw an electrician wire a motor at 120 volts and 240 volts, the same motor. At 120 volts, the motor used about 10 amperes and at 240 volts it used about 5 amperes. That tells me that the formulas for electrical power are probably right. It also tells me you didn't read my responses. The thing is that someone said I wasn't reading them but I did. It is my question that is not being understood.

As for the units for mechanical power, they are what they are because physics defines mechanical energy in units as Newton x meters or kilogram x meters per second x meters per second. So who decided that it should be that way? That is all I'm trying to find out and no body seems to be able to answer. Telling me that the scalar versions of impulse and momentum have different units is not helpful. I already know the units are different but that does not mean they are wrong if no one ever verified mechanical energy just as it says to do with the scientific method.

I know impulse and momentum cannot represent energy but that does not mean that the scalar versions of them could not. Isn't energy scalar? So, why can't the variables in a scalar concept be scalar?

I don't like it when people criticize me for asking questions. I only ask because I don't know. How can it be wrong to find out why something is a certain way? Isn't that what a scientist is supposed to do? Why is no one answering the question?

I think if we found out that the work energy theorem was wrong that it would be a wonderful thing for physicists. I'll bet there would be all kinds of opportunities to re-write books, and make discoveries. It would be good for physics but it would be even better for me if I just found out why work is related to force x distance instead of force x time (scalar).

Why won't anyone answer my question? Does anyone know who came up with the idea and who did the experiment that showed everyone it was so? I hope my teacher, whoever he might be knows, because no one is helping me.

One last thing before I check on dinner, why am I the only one talking about the scientific method? Yeah, why? I may not have any degrees but I do know that physics is supposed to be about the scientific method. I don't care if you all accepted the work energy theorem without proof. That don't matter a lick. But I want to know.

 

[Nugatory]

Why won't anyone answer my question?

I thought I gave it a pretty good try back in #25 in this thread... If you could tell me what part of it is not responsive I might understand your question better.

 

[russ_watters]

I explained earlier that I saw an electrician wire a motor at 120 volts and 240 volts, the same motor. At 120 volts, the motor used about 10 amperes and at 240 volts it used about 5 amperes. That tells me that the formulas for electrical power are probably right.

That tells you volts*amps is conserved: it doesn't tell you that it's power.

As for the units for mechanical power, they are what they are because physics defines mechanical energy in units as Newton x meters or kilogram x meters per second x meters per second. So who decided that it should be that way? That is all I'm trying to find out...

It isn't really true that that's all you are trying to find out. But if all you want is a name, try Gottfried Leibniz. His idea was most of the way to what is the modern version of energy (he almost got it right) and is the originator of the basic concept. The wiki has some history:
http://en.wikipedia.org/wiki/Energy#History

Telling me that the scalar versions of impulse and momentum have different units is not helpful.

It should be. I recognize that you are at the beginning of your journey with physics, but one of the first things you need to learn is that with math, the units on both sides of an equation must match.

I already know the units are different but that does not mean they are wrong...

They are wrong if you try to put it into an equation where they don't fit. That's a basic concept in math. It doesn't mean they aren't valid for other, different equations, though. Like momentum = mv. But having momentum be useful doesn't mean energy isn't and vice versa -- and again, you already acknowledged that you know that it works. And it doesn't mean you can plug energy into a momentum equation and have it work. It won't.

...if no one ever verified mechanical energy...

You need to take a step back and consider just how asinine that statement is -- an also go back and read through the thread to all of the examples already provided of how it gets verified. There's been half a dozen at least so far.

I know impulse and momentum cannot represent energy but that does not mean that the scalar versions of them could not.

It does if it means that when you plug them into the energy equations, the equations don't work.

I don't like it when people criticize me for asking questions. I only ask because I don't know.

We don't like it when people ask questions and then don't try hard enough to understand and accept the responses.

How can it be wrong to find out why something is a certain way?

It isn't. But not accepting the answer is wrong. Saying you want to learn and then fighting against learning is wrong.

Why is no one answering the question?

We don't like it when people ignore our answers and then say no one answered. There are dozens of quality answers in this thread and you haven't directly addressed (by quoting the relevant part) any of them.

I think if we found out that the work energy theorem was wrong that it would be a wonderful thing for physicists. I'll bet there would be all kinds of opportunities to re-write books, and make discoveries. It would be good for physics but it would be even better for me if I just found out why work is related to force x distance instead of force x time (scalar).

Because you are a kid, I'll say that's cute. But you need to recognize that because you are a kid, that's all it is. Coming from an adult, it would be laughable for you to think that the first thing you learned in physics is something that every physicist and engineer (millions of them) for hundreds of years got wrong.

One last thing before I check on dinner, why am I the only one talking about the scientific method?

Because you don't yet know enough about science to start learning the scientific method. What you are asking and not understanding over and over again is more basic even than the scientific method is. Your issue is essentially a communication issue: you have to learn how to talk about science before you can learn science.

 

[Dale]

why am I the only one talking about the scientific method?

You are not. I told you why your question was scientifically wrong back in post 2. What you are complaining about is simply not science.

In the theory part of science you make assumptions. If you are not making assumptions then you are simply collecting data without any theory and therefore you are not doing science.

Also, in any field of study, including science, you define terms. There is no proof or justification or faith or anything else that you have mentioned involved in defining terms. A definition is simply a convention, and like all conventions it could be different, but we use the agreed upon convention because it makes something easier. In the case of definitions, using the convention makes communication easier.

In science, if you ASSUME Newton's laws and rigid bodies then you can prove the work energy theorem. (There is no chance for it to be wrong without all of math being wrong also). From the work energy theorem you find that $f\cdot d=\Delta(\frac{1}{2}mv^2)$. For convenience in communication we define "work" to refer to the quantity on the left and "kinetic energy" to refer to the quantity on the right. There is no proof or experiment involved.

Now, you can perform experiments and measure the relationship between work and kinetic energy. If you do that and get a result other than their equality then you have proven either that the objects are not rigid or that Newton's laws are wrong (or your experiment is flawed).

However, no matter how much experimental data you collect disproving the work energy theorem, you cannot ever even in principle prove that $W \ne f\cdot d$ nor $KE \ne \Delta(\frac{1}{2}mv^2)$ because those are tautologically true by definition.

 

[speedingelf]

No. You have to make assumptions. That is part of science. You make assumptions and then you test those assumptions against experimental evidence.

I am doing science, it is you who are not. If someone assumed that there was this called energy and came up with the work energy theorem, the scientific method requires checking any testable predictions.

What am I doing? I'm asking to see that experimental evidence. If that is not science, then I don't know what science is all about. What am I supposed to do, just accept it because everyone else does. I'm sure Galileo who showed that heavier objects fall at the same rate as lighter ones would take my side in this matter. Don't you?

 

[speedingelf]

In elastic collisions there are two conserved quantities: A vector whose magnitude is $mv$; and a scalar $mv^2/2$. That's the experimental fact that we have to start with.

We label one of them momentum (because we have to name it something) and the other kinetic energy; and we define force to be the time derivative of momentum because that's convenient and agrees with the experimentally confirmed result $F=ma$. It follows from this definition that we apply a given force for a particular length of time to change the momentum by a given amount - that's just doing math on the relationships we already have.

Now, if we turn our attention to that other conserved quantity, the scalar $mv^2/2$, and ask the question "What application of a given force will change the quantity $mv^2/2$ by a given amount... a bit of algebra and an easy integration will give you force times distance.

The discovery of kinetic energy and momentum, and the relationship of kinetic energy to potential energy (which comes directly from $W=Fd$) didn't follow exactly this path, of course. The history is full of misconceptions, false turns, and a even people expressing confusions similar to yours. It is only when we arrive at the end of the journey and look back that we are able to say "Oh - so that's the shortest route with the fewest wrong turns", and start writing the textbooks.

I understand what you are saying. I even did a bunch of collision problems myself and even worked out the formulas to do calculations for ideal elastic collisions.

I am not confused; all I want is the answer to a simple question.

I'm having trouble getting anyone to understand what I am after. We have the work energy theorem and it does have a testable prediction. According to the scientific method, whenever a hypothesis is proposed, it has no scientific value until it is confirmed experimentally. Today, I know there are scientists working on String Theory and they might even be teaching it, but String Theory is really only a hypothesis; it has not been proven. If it was, I'm sure that it would have been on the news. Don't ask me about it because I know very little beyond the fact that it is supposed to require more than 3 spatial dimensions. The point is, that today's physics community will not go out on a limb and grant String Theory (it should be called the String Hypothesis) the status of something like the Theory of Relativity.

The work energy theorem is at least 100 years old, probably a lot older, and if it has never been tested, that fact should be made known. I'm seriously beginning to believe that no scientist has ever tested the work energy theorem without assuming it is correct. The air track experiment suggests that. Is there some secret provision in the Scientific Method that says that something everyone agrees is true does not need experimental proof. Is there a provision that says anything older than a certain date does not require proof?

I don't understand why a simple question has generated all this noise. Am I being a b|^(h for wanting to know? It is as if the scientific method does not apply to students and the thing to do is to say all kinds of irrelevant things so the student forgets about the question. I really don't get why no one will answer the question of who came up with the work energy theorem and who proved it was right? Is it a secret that is only told to doctoral candidates or what? I'm getting a little upset right now and so, until tomorrow.

 

[russ_watters]

Ok, I think that's about enough. You've made it abundantly clear you don't want the answers you are being given, so this thread is closed. If you change your mind, my offer in post #40 still stands. In either case, if you ever decide to change your approach, please try rereading this thread and try to understand what you are reading instead of knee-jerk dismissing it.