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Why do we use the physical quantity work to measure energy?

  1. May 8, 2016 #1
    1. The problem statement, all variables and given/known data
    To me it makes more sence to use force*time instead of force*distance. Because when you are aplying force to an object that isn't moving you're still wasting energy. Or when you push a weight over the same distance with little force or big force, the amount of energy wasted should be the same, but it isn't because the power is different...

    3. The attempt at a solution
    Maybe I don't understand the definition of work, but why is it measured in joules then?
     
  2. jcsd
  3. May 8, 2016 #2

    CWatters

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    Ok so consider a book sitting on a shelf. The book can exert a force on the shelf for many years.

    If work = force * distance then the work done by the book on the shelf is zero, because the book doesn't move.
    If work = force * time then it appears the book is doing work on the shelf. Where does the book get the energy to do that? What does the shelf do with the energy?

    Why are you concerned with the energy "wasted"? Concentrate on the useful work done first. In both cases you have moved a box the same distance against the same friction force - so the energy converted into useful work is the same.

    What is different is the power. Power = energy/time. So moving the box faster requires the same energy but greater power.
     
  4. May 8, 2016 #3

    CWatters

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    PS..

    The human body isn't very efficient and it's easy for this to confuse you. For example it takes no energy to hold a heavy weight out in front of you. If you hold a weight at a constant height then you aren't doing any work on the weight. However it may feel like you are doing a lot of work and it may get harder and harder to keep the weight from falling. This is entirely due to the way the human body works and it's inefficiencies. If you put the weight on a shelf the shelf doesn't need an energy source to hold it up. It doesn't have to eat, breath and turn food into fuel/energy to hold the weight up.
     
  5. May 8, 2016 #4
    The object may not be moving, but your muscles are moving. Holding an object up, for instance, requires that the cells in your muscles contract. But no cell can hold a contracting position forever, so your body has an ingenious scheme whereby only some of the cells in any particular muscle contract at any given moment (it is usually between 1/3 and 1/2 of the cells contracting at any given time, if I recall correctly). This allows the cells to have rest while you continue to do whatever. Of course, eventually the chemical stores are depleted and the muscle must rest. Thus the constant contracting and relaxing of individual muscle cells means they are doing work and producing heat. So you get tired even though you didn't move the object.
     
  6. May 9, 2016 #5
    But moving the box faster requires a different energy, because for a faster movement you're using more force . Which acording the the work formula (W=F*s) leads to more energy???
     
  7. May 9, 2016 #6

    jbriggs444

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    The force of friction (e.g. of a box on the floor) does not increase with increasing speed. Accordingly, it does not require a different amount of energy to move the box across a distance rapidly rather than slowly.

    The required energy will need to be delivered in a shorter period of time, however. So it will take more "power".
     
  8. May 9, 2016 #7

    SammyS

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    Perhaps it would be a better idea to consider a technical description of energy. (It's difficult to give a short concise definition.)

    From Wikipedia:
    In physics, energy is a property of objects which can be transferred to other objects or converted into different forms.[1] The "ability of a system to perform work" is a common description, but it is difficult to give one single comprehensive definition of energy because of its many forms.[2] For instance, in SI units, energy is measured in joules, and one joule is defined "mechanically", being the energy transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton.[note 1] However, there are many other definitions of energy, depending on the context, such as thermal energy, radiant energy, electromagnetic, nuclear, etc., where definitions are derived that are the most convenient.
    ...​
    In particular, I have used the phrase:
    The "ability of a system to perform work"​
    given in that snippet, in physical science courses and in Liberal Arts physics classes.
    .
     
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