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About Work and energy

  1. Dec 10, 2011 #1
    I've always seen Work as an extra mechanic energy given to the system... but i really cant figure it out :

    If we lift a 2kg ball, that's steady, using g as 10m/s², with 25N, we'll have a 5N up resultant force....

    after its 5meters up, the resultant work will be 25J (5N.5m), but the potential energy by itself will be 100J (20kgx10m/s²x5m) and the kinetic energy will be 25J (i found it using the F=ma to discover de aceleration and then using torricelli's formula..)..

    How can I give 25J to a system and it become 125J?

    If the same ball were dragged along the floor, with 25N, and a friction force of 20N , after 5 meters, the resultant work would be 25J, the same as the extra mechanic energy ... can someone help me?
  2. jcsd
  3. Dec 10, 2011 #2


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    you still used 25N to lift, so the work done by you would be:

    25 N*5 m = 125 J

    From that 125 J of work, 100 was stored as potential energy, the other 25 is being used as kinetic energy (since the object is still moving).

    If you are dragging a ball against friction, you are doing work against the friction, so the work is 45 N * 5 m = 225 J.

    I don't know what "extra mechanical energy" means.
  4. Dec 10, 2011 #3
    But shouldn't I use the Resultant force?

    as if it were horizontally and the Gravity force were the friction force?
  5. Dec 10, 2011 #4
    well, the friction will do a work of -100J (once its againts the movement) and the force will do a 125 work... the work will be 25, as if it were a 5N force only... I learned that way, is it wrong?
  6. Dec 10, 2011 #5


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    It depends on the exact question. If you're asking how much work you do to pull it along the ground then you are doing work against friction, so that must be added.
  7. Dec 10, 2011 #6


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    Well no, the wording you've used here makes it right. You specified which force did how much work and used the qualifier "as if". So your statements here are correct.
  8. Dec 10, 2011 #7
    I think I got it.... when it comes to friction, the energy becomes heat, so we just cut it off the equation to calculate the Mechanical Energy...

    but when it comes to Gravity force, the energy becomes potential energy, so we add it to the equation.... is it right?
  9. Dec 10, 2011 #8


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    Yes, that's true. Gravity is a conservative force, friction is not.
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