According to various sources that i have come across, energy is the ability to do work. Can someone elaborate how diff forms of energy like potential energy and kinetic energy are ability to do work? I have tried searching but all i have found are the conservation of energy and diff forms of energy. Energy is such an important topic but all that is talked about are formulae of various potential energies and forms of energy and conservation of energy.
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$\begingroup$ Perhaps there is nothing more to say. Potential energy is defined by the work done by a conservative force. And Kinetic energy by the work energy theorem. What's wrong with that? Moving objects can give energy to other objects via collisions. $\endgroup$– user196418Commented Dec 27, 2018 at 17:03
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2$\begingroup$ This is such a nice question, but unfortunately, the answer might not be very satisfying. The definition of energy is itself questioned. We all work with it, but we wouldn't know how to define what it is. It's just some quantity that is often conserved in systems, but there's much discussion on how to define it. You've beaten me because I think it backwards: "work is a measure of changes in energy", being energy the primordial thing. Hope this is not a circular argument haha. $\endgroup$– FGSUZCommented Dec 27, 2018 at 18:50
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$\begingroup$ @FGSUZ I was thinking exactly the same thing: "work is a measure of changes in energy." I go even further and say that "work is a transfer of energy from one system/object to another." Energy is more fundamental than work, because energy is conserved while work is not. BTW, I hate the "ability to do work" definition because it leads to questions like this; the definition is terribly flawed, but books keep using it. $\endgroup$– Bill NCommented Dec 27, 2018 at 23:17
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$\begingroup$ Examples: 1) Kinetic energy does work when a bullet strikes a target. If the bullet expands on entry, such that it stops in the target, it does maximum work on the target via the work/energy theorem; 2) Water behind a dam has a lot of gravitational potential energy due to its height, and this potential energy can be turned into the kinetic energy that turns a generator and generates electricity as it exits the bottom of the dam; 3) Electrical potential energy in flashlight batteries can be turned into light and heat when the flashlight is turned on. $\endgroup$– David WhiteCommented Dec 27, 2018 at 23:30
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$\begingroup$ Now I see one of yoru sentences again. Different forms or energy are not able to do work. Forces do work, energy does not. Work is change in energy, and it is caused by forces. Energy and work are kind of like position and velocity. Work (change of energy) does not depend on energy. $\endgroup$– FGSUZCommented Dec 27, 2018 at 23:51
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3 Answers
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It may be useful for you to think of energy as work which has been stored up for future use. Here are some examples:
If we hoist a massive object upwards in a gravitational field to a certain height, we must perform work in the amount (work) = (mass)x(gravitational constant)x(height). That work is now stored in the system as potential energy. If a rope is tied to that object and also wrapped around a wheel, releasing the object will pull the rope and turn the wheel which could then set a machine into motion to perform work for us.
If we squeeze a coil spring's ends together through a distance l and hold it in that squeezed position, we have performed work on the spring in the amount (work) = (1/2)x(spring constant)x(l^2). that work is now stored up as elastic potential energy in the spring and if we stuck that spring between two blocks on the ground and released the spring, it would perform work on the blocks by pushing them apart.
answered Dec 27, 2018 at 21:52
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I think that trying get a full and accurate description of what energy actually "is", is a slippery subject. But your question might, I think, possibly be answered in this post: What is Energy?,
Articles such as Wikipedia: Energy tend to be a collection, as you imply in your question, of lists of different ways in which we categorize energy by involving the source involved, nuclear, thermal, solar etc.
My own way of thinking about it is to view kinetic energy as the energy that actually does work, ultimately by electromagnetic attraction and repulsion in most everyday situations and processes.
I think of potential energy in two ways: as a (limited) source for the kinetic energy associated with it (which actually does the work), and also as a bookkeeping exercise used in our calculations to ensure that the law of conservation of energy is maintained.
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Energy isn't the ability to do work. It's possible to have a system with a great deal of energy but it's difficult to get any work from it.
It's because of the conservation of energy that energy as a physical concept has become so important.
Can someone elaborate how different kinds of energy like potential energy and kinetic energy can do work?
If you put an orange on top of a table it has potential energy but it is not doing any work. A comet moving in empty space has kinetic energy but not is it doing any work.
Work is defined as a force moving through a distance. Change needs to occur, in the sense of forces being applied, and distances moved through for work to happen.
answered Dec 27, 2018 at 23:33
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$\begingroup$ I think you are confusing "ability" with "actually doing". If something has a high kinetic energy, then it has an ability to do a lot of work on something, even if it never happens. If an object has a lot of potential energy, then it has an ability to do work on whatever it interacts with to form that potential energy. I agree that the statement isn't perfect, but I wouldn't dismiss it outright. $\endgroup$ Commented Dec 28, 2018 at 6:00
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$\begingroup$ @Aaron Stevens: Not at all - it's the difference between potentiality and actuality. Potential energy was named after this sense. $\endgroup$ Commented Dec 28, 2018 at 13:47
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$\begingroup$ Right. Having the ability (or potential) to do something is not the same thing as actually doing it. Which is why a fast and loose definition of energy is the ability (or potential) to do work and not actual work being done. $\endgroup$ Commented Dec 28, 2018 at 13:59
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$\begingroup$ @Aaron Stevens: So why are you arguing needlessly? $\endgroup$ Commented Dec 28, 2018 at 14:01
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$\begingroup$ I'm just trying to understand. Your answer completely dismisses the idea of energy as the ability to do work, but then in your comment you say that there is a difference between potentiality (ability) and actuality, which means that you do find some merit in the idea of energy as an ability (potential) to do work. $\endgroup$ Commented Dec 28, 2018 at 14:06