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Explaining the Concepts of Force and Energy

  1. Apr 30, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    1. The dilemma
    Work (change in energy) is defined as the exertion of a force over a distance parallel to the force.

    So if force is exerted, but nothing is moved (presumably with two immovable objects), then no energy is changed.

    But think about the dilemma with a person lifting 100 pounds above his head. He exerts a force of 100 pounds to counter the weight, and say he keeps it above his head for 1 hour. According to the work equation, the person has changed 0 energy in doing this act.

    But he has continually exerted a force of 100 pounds for 1 hour, and now he is very tired. But according to work, he has spent 0 energy.

    What am I missing?
     
  2. jcsd
  3. Apr 30, 2017 #2
    Another question:

    Let there be an object of mass 1 kg in space. There is a constant force F1 acting on the left and F2 acting on the right. F1=F2. The object observes no acceleration. But the object was moving in space at a constant velocity of 1 m/s to the left.

    The object moves for 10 seconds.

    How much work has F1 done on the object?
     
  4. Apr 30, 2017 #3

    PeroK

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    He's not doing any external work and, if the weight wasn't too heavy, he would only get tired very slowly compared with lifting the weight repeatedly.

    But, it takes internal (biomechanical) energy to keep muscles in a locked position, Also, this work is dependent on whether the muscles are in a comfortable position.

    If you put the weight on a table, the table wouldn't get tired supporting the weight and wouldn't need a fuel supply.
     
  5. Apr 30, 2017 #4

    PeroK

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    I'm sure you can work that out for yourself.
     
  6. Apr 30, 2017 #5
    That is true that it is much less tiring than exerting a force greater than 100 pounds to lift it up and down repeatedly. And your point about a table needing fuel makes a lot of sense.
     
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