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I have a question about the definition of energy. the definition is "ability to do work". what does it mean? what is the ability that the definition talks about? what is the meaning of ability in energy? I understood the concept of work, and it would be great if someone could explain to me the concept and definition of energy.

Qmechanic
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R24698
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3 Answers3

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My favourite example of using the definition: Why do we say that a body of mass m moving at speed u has kinetic energy of $\tfrac12 m u^2$ ? Imagine this: a cart of mass m moving at speed $u$ on level ground has a rope trailing from it. Someone grabs hold of the rope and exerts a constant retarding force -F on the cart, bringing it to rest in a distance s, while being pulled a distance s himself/herself. The cart's acceleration is $-F/m.$ So using $v^2=u^2+2as,$ we have $$0=u^2+2\frac{(-F)}{m}s.$$ So$$Fs=\tfrac12 m u^2.$$ But the cart exerts a force $F$ on the person via the rope, so does work $Fs.$ The cart's ability to do work was $\tfrac 12 m u^2,$ because that is the amount of work it could do!

Philip Wood
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  • @Phillip, +1, I like your derivation of the connection between Work and Kinetic Energy! Initially, I didn't understand why you used $v^2=u^2+2as$, as I had forgotten that $v$ corresponds to the final velocity and $u$ corresponds to the initial velocity of the mass. I found a demonstration of the use of this equation here. You set $v=0$ because that was the final velocity. Substituting $- \frac{F}{m}$, showing that Work equals Kinetic Energy was an unexpected surprise! – Thomas Lee Abshier ND May 11 '19 at 02:02
  • Good! In fact one doesn't need to assume a constant force or straight line motion; more sophisticated derivations use vectors for displacement, velocity and force. They also use some elementary calculus. – Philip Wood May 11 '19 at 07:10
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Roughly speaking, work is the transfer of energy from one thing to another due to the application of a force through a distance in the direction of the force. In order for something to be able to transfer energy to something else, it must possess, at minimum, the amount of energy transferred for conservation of energy.

Thus something that possesses energy is capable of doing work.

In response to your follow up question:

but what ability means?

Ability (or capability, or capacity) means, in this context, the potential for doing work, that is, the potential for transferring energy to something else by means of force acting through distance. In order for something to have this potential, it must possess energy. Therefore, the possession of energy means having the ability to do work.

Hope this helps.

Bob D
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  • Bob D, your definition is true, but there is a real subtlety involved that deals with whether or not an object is in its lowest energy state. Energy, conservation of energy, and work, are often very abstract subjects that can't be unambiguously described very easily. – David White May 07 '19 at 23:20
  • @DavidWhite I absolutely agree. For example, see my comment to SR810, pointing out that not all energy possessed by something is necessarily available to perform work. – Bob D May 07 '19 at 23:25
  • Bob, I saw the comment and upvoted it. And note that I have the opinion that it's a "shame" that English and all other human languages, are in fact ambiguous, which means that an English description of physics concepts is also ambiguous. I had to deal with this problem when trying to teach AP Physics 1 to high school juniors. – David White May 08 '19 at 02:59
  • but what ability means? – R24698 May 09 '19 at 14:05
  • @RoeeTabak See my revision to answer your question. Hope it helps clarify things. – Bob D May 10 '19 at 12:08
  • @DavidWhite Regarding your comment on, what I will call, the interference of language on learning physics, I absolutely agree. When I taught review classes for the Fundamentals Exam (FE) for the PE license to multilingual participants, I tried to focus on mathematics and pictures (drawings) as opposed to words (in english, which I am limited to). – Bob D May 11 '19 at 22:54
  • @BobD, I'm not surprised that you agree. In fact, when I occasionally taught beginning thermodynamics, I warned students that English tended to get in the way. I even commented "heat is not hot, it's energy" ... I'm not sure they completely understood what I was saying but I am sure that their prior invalid concepts got in the way. – David White May 11 '19 at 23:59
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Energy is like a reservoir, which has been loaded with some flow of force say filled like a pond with work. It could give back that reserved work, in analogy with the pond some or all of its water.

But how much of it you can actually tap into has to do with many factors.

Again in analogy to the pond, you have to be at a lower level and also have enough room for all the water or energy.

Earthquake releases much energy, but a cardboard box can hardly take any of it, while a massive high-rise takes a lot.

Huge water reservoir behind a dam has much potential energy but if you want to tap it into a turbine you need to let some of it waste as the flow kinetic energy through the turbine.

So energy is capable of doing some work, but it needs means to deliver that work.

kamran
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