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Positive and negative work in terms of energy.

  1. Jan 8, 2016 #1
    How would we define positive and negative work in terms of energy? When the force and displacement are in opposite directions, we say the work done by the force is negative. When the force and displacement are in the same direction, we say the work done by the force is positive. However, how we would define the two in terms of energy. As far as I have understood this concept, I believe that whenever a force is supplying energy to a body(the work done gets converted into kinetic energy), it is positive work. But whenever, the force is draining the body of its kinetic energy and converting it into other forms (like thermal energy), it is negative work. Is this correct? If it isn't, what is the correct way to relate energy and work. And can someone please elaborate this topic? Thank you very much.
  2. jcsd
  3. Jan 9, 2016 #2


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    Energy exchange is like a financial transaction: Someone's credit is inevitably someone else's debit. Trying to define what is ultimately positive and negative is pointless.

    In every situation work is positive from one point of view and negative from another. Or, at least, that's a better way to start thinking about it.
  4. Jan 9, 2016 #3
    What if we had to analyze whether a particular force was doing positive or negative work?
  5. Jan 9, 2016 #4


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    Work is defined to be positive or negative as you have described. ##W = F.dr## If overall energy is conserved in a system, then it follows that the work ##W##, whether positive or negative, must be adding energy to one part of the system and subtracting it from another.

    That was my point. The work gets added to one component of the total energy and subtracted from another.

    I think you understand all this from your first post, which I may have misunderstood. I thought you were trying to equate postive work with positive energy. In terms of the KE of an object subject to a force, it's true that positive work equates to an increase in KE and negative work to a decrease.

    Rereading your post, I'm no longer sure what your question is! In terms of forces, work and KE I think you understand that.
  6. Jan 10, 2016 #5
    This is the answer I was looking for. Thank you.
    I basically wanted to know what negative and positive work are in terms of energy loss and gain.
  7. Jan 10, 2016 #6
    let's imagine that we are on the tennis court.
    Tennis gun shoots two balls almost simultaneously. Let the first ball flies over the net almost touching it.
    Let the second shot a little weaker than the first. This leads to the fact that a second ball hits into the net. We see that the second ball loses energy and hence his work is negative. Ant sitting on the first ball sees that the the net hits the second ball, and it has accelerated. This is because before the hit both balls were fixed relative to each other. So, the ball has incrised its KE from the ant's point of view.
    This means that it is necessary to recognize the relativity of any sign of work or abandon the theory of relativity. And the second way is also a possible option!:biggrin:
  8. Jan 10, 2016 #7
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