Definition of Energy (Forces) : LUSENET : Middle School Science : One Thread

Energy is defined as the ability to do work. Work is defined as exerting a force over a distance. My question is, why would it be incorrect or incomplete to simply define energy as the ability to exert a force? Yes, I understand that the distance part of the definition is crucial to the concept of work, but isn't energy required any time a force is exerted, even if the force doesn't result in the object actually moving? Here's a concrete example of what confuses me. A book sitting on a table is exerting a force (weight) on the table. But this force does not result in either the table or the book moving because it is balanced by the "normal" force that the table exerts on the book. No work is done. By definition, it seems, no energy is required for this interaction between table and book. But how can the table exert this normal force without energy? If it needs energy in order to exert this force, doesn't "the ability to exert a force" satisfy the definition of energy? I know that I must be mistaken somewhere in my understanding of forces and energy. I'm trying to figure out where that is.

-- Michael Gatton (, June 27, 2000


A possibility...

Does the defintion apply to an idealized situation, i.e. one without any forces counter-acting your exertion of force? If so, than any time you exerted a force, the object would move (pushing a car in outer space for example). If you attempt the same task on earth, the forces of gravity and friction "push" against your exertion of force and the object does not move. If this is the case, then a "real" definition of energy must include variables that account for such counter-acting forces. Just a guess.

-- Jonathan Brenner (, June 27, 2000.

I think of energy as "a measure of the ability of one object to change the state of another object." There is always a reciprical relationship between the object that causes change and the object being acted upon. This relationship can be examined in terms of of applied forces and counter forces. When applied and counter forces are balanced, energy is not exchanged, but the potential for change exists. Energy is exchanged when the magnitude of the applied forces exceeds the magnitude of the opposing or counter forces. A change in state occurs when this happens. The measure of this change is related to the magnitude of the resultant forces( sum of the forces in the x and y directions) and the distance over which those forces acted. This exchange simultaneously affects the object causing the change and the object whose state is being changed. The object causing the change may slow down or cool off, the object being acted upon moves, speed s up or heats up. So energy is really an arbtrary measurement of the final result,(or possible result)when a change in state occurs as a result of the interaction between two (or more) objects.

Just my opinion.

-- Bruce W. Gerhardt (, February 09, 2001.

I recently had the chance to discuss some of the questions on this forum with a retired high school physics teacher who prefers to remain anonymous for now. I will try to summarize his points here. If this e-mail doesn’t make sense, then come to the forum and read the whole thread for context.

I had come to the conclusion that my problem understanding the ‘definition of energy” ultimately had to do with the nature of forces. I asked about the force holding the book on the table and whether energy was required for that force to keep the book from falling to the ground. When it comes to the fundamental forces in the universe (strong & weak nuclear forces, electromagnetism, and gravity) the answer is “no.”

It doesn't take energy to apply a (fundamental)force...The book stays on the table, because the table stays together held by chemical (really electrostatic) forces.

I also asked whether these forces would still exist if and/or when the universe runs out of energy:

Finally, the answer to: "If the universe were to "run out of energy" would those forces still exist?" is yes. In the “heat death” of the universe -- don't hold your breath waiting for this, there would still be energy, just not any that we could extract.

I also got this pearl of wisdom about the limits of our understanding:

Let's talk about the atom. Maybe something as simple as the hydrogen atom. One proton, and one electron. In some sense, protons just attract electrons. That is the nature of the electrostatic force. Science, is much like the game of "why" that kids like to drive their parents crazy with. However after a certain number of questions, the parent ends up saying. "It's because I said so." In science the answer is, "Because that is what our experiments tell us."

So, the ability to apply a force DOES NOT satisfy the definition of energy. Back to “the ability to do work.” So what is work? The formula for work (its mathematical definition) in physics is: work = force X distance X the cosine of the angle between them. Let’s forget about the cosine for now, and concentrate on the force and distance part of the equation. If no force is applied, then no work is done. If there is no displacement (distance = 0)then also no work is done. The cosine of the angle between them is beyond the middle school curriculum but interesting nonetheless:

The cosine in the formula implies that if the force is perpendicular to the way the object is moving (the cosine of 90 degrees is zero) then no work is done either. Thus the force of gravity that holds a satellite in a circular orbit is also not doing any work.

Any Questions?


-- Michael Gatton (, March 01, 2001.

A simple definition of energy reads: the capacity to do work by moving matter against an opposing force. If neither the table nor book moves, no work is done. This means that the work produced by gravity pushing the book down plus the work produced by the table pushing the book up equal zero. In the same sense, the total amount of energy exerted in the system equals zero.

-- Ben Snyder (, October 15, 2001.

this is all boulderdash

all of these definitions is not explaining what energy is. the definition should be somethink like. energy is made of mass which is part of the body of matter.

note that i say mass because this massless partical stuff is baulderhash too. for something to be detectible, it must have mass. for something to be... it must be made of something. because it is too smale to see or detect, does not mean that something is not there.

about the force working against or exerting the force is not correct. energy is not about the working of energy, but what it is, the definition of why we call energy, energy and not the working of how energy flows, which is what this force stuff it all about. there is more to just force, what about wind? this definition fall short.

the definition of energy goes something like this... energy is a compound of matter which is smaller then an atom and travles at the speed of light. nuff said

-- earnest_hern2000 (, January 21, 2002.

At the middle school level, we only deal with classical or Newtonian physics. Einstein's theory of relativity, energy/mass equivalence, E=MC^2, etc, are just introduced in late high school. The classical definitions of mass and energy and force and work are still valid for all but special circumstances (like particle accelerators). Of course, I realize that relativity could be invoked to answer my original question concerning forces and energy, but I'm satisfied that the question has been answered above in Newtonian terms.

For those of you who are more interested in the topic, check out this discussion in the Stanford Encyclopedia of Philosophy.


-- Michael Gatton (, January 22, 2002.

Ok. I was just wondering why Einstein's information was not here. This is a great site. Bye now. O... you can delete my above post if you like because it is not related to the information at this site.

-- earnest (, February 25, 2002.

Do not confuse 'energy' defined above with 'human energy': if you try to push a car with the brake applied, you definitely get hot and tired, while the car has not moved an inch. You might think then that applying force to the car alone 'costs' energy. (The energy it costs you to push agains a non-moving car is energy needed to move your arms, feet, stretch muscles, breathe, etc)

Force and energy are just a physical definition in a model that eases 'understanding' of the world around us (scientists are happy if they understand the world around us, or at least think they do). With the model we can explain (or mayby define) things like energy conversion, conservation of energy. Or more practical: how much current and voltage we need for a light bulb to light up the room or how strong the WTC has to be to resist impact of an airplane.

Why simply exerting a force does not 'cost' energy does not fit with this model. If it 'costs' energy just to apply a force is not consistent with, for example, 'conservation of energy'. Assume exerting froce would cost energy. The book on our table and the table itself would cool down for example, simply because the energy to apply the force has to come from somewhere. If the book and the table would reach absolute 0 temperature, the book starts to hover, since there is no more energy to push it on the table. Since we do not experience this in 'real' life, it does not make sense to assume exerting force costs energy.

Note: in the physical model nothing 'costs' energy. 'Costing energy' means: some energy is converted into another type of energy. No energy is lost.

-- Jorgen (, September 24, 2003.

Your questions are very mined full and interesting. I agree that there should only be one definition of 'Energy'. I recon your definition is very technical. The answer to your question 'Will there still be these forces when the universe runs out of energy' from my piont off view they will be because there will always be a sun and if the sun burns out well i'll be damed. thanx for reading and if you send me anything on email it probly wont werk

-- Jessica Frost (, October 01, 2003.

It's rather a hippie-like theory of mine. It is known that nothing can be destroyed in the universe, even energy can only be put into a different form at the most. A black is the supposedly the opposite of existence, but the energy does still exist only in a different relative form than this dimension. take all elemental enery for example. The sun would be nothing without it's gasses and mass to keep it going. Basically energy comes from the vibration of molecules at the smallest level. Viewing energy as a separate entity then mass would negate it because without the mass of the molecules to generate further vibration, energy becomes dormant. But even dormant energy is just energy without mass to further it's vibration. So the real question is - is all existence the vibration of one molecule echoing in space/time, or is time energy and we are merely the mass living within it, because when we run out of time we run out of energy and cease to exist on this "plane". Any further comments will create some huge argument about God and blah blah blah. Just a thought.

-- Ben Althauser (, October 25, 2003.

I made a typo. where it talks about "a black" it should be "a black hole".

-- Ben Althauser (, October 26, 2003.

I have a similar (maybe a little more complicated) question. Let's say that a rocket stops accelerating and the propultion from the engine its used only to make it float within the range of gravity. The work and the energy produced by the engine are measured in Jules. Since Work (J) is zero, therefore Energy (J) would be zero. We know energy cannot be zero because the engine would produce heat energy. But how can we calculate how much energy is produced?

-- Ramon Salazar (, March 25, 2004.

In my efforts to understand energy in the simplest terms possible, I find that I am hard pressed to think of any form of energy that does not require or involve movement. Even potential energy is defined by the potential to move. Heat energy is related to molecular movement, and so on. This seems to work for me. Anyone aghast at this view should e-mail me directly as I am a middle and elementary school science teacher and should be set straight. Just be nice.

-- Treena Joi (, April 04, 2004.

There is a very good discussion of this in Richard Feynmann's /Six Easy Pieces/. He begins by (thankfully) admitting that we don't really know what energy is, exactly, but we do know how to measure it.

Treena (sp?) is basically right, but it get's more complicated as you look at 20th century physics.

"The ability to move stuff" covers most classical cases: things moving fast can whack other things and make them move, something up high can fall, gasoline can explode and push a piston, and so on. These guys have energy and they're able to move stuff. And if you think of heat as the motion of tiny little things, then motion and heat are the same thing, and you have one less form of energy to count.

But it get's more complicated when you talk about light. Light carries energy in proportion to it's frequency. To a 19th century physicist like Poincare, this is the frequency the Ether is wiggling, so you could think of it as a kind of motion. But we don't believe in the ether any more, so it's no longer clear what's moving exactly. There's this photon, and it "carries" energy. How it carries it, or what it is exactly, we don't know. But we do know how to measure it: it's the frequency times h (Planck's constant). And when the light strikes an electron, the electron "jumps", the atom has more energy, the substance get's hotter, the water in the microwave boils, and we can see the steam moving, so it can turn back into motion, eventually.

-- Charles Gillingham (, April 14, 2004.

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