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Work and energy

  1. Apr 23, 2005 #1
    HI! I have a problem here about work and energy. !!!Help is needed!!!

    "If a spring (on horizontal surface) is pressed down by 10N force the max. compress is 1m. What is the : a)Work done b)Potential energy gained by spring?"

    First of all I found 10=k x 1 k=10N/m

    Then work done W=F.x=10.1=10C

    Ep=kX^2/2=10x1/2=5C

    As a result Energy gained is half of the work done.Is it correct? Where is the other half of work? :smile: :smile:

    THX! THX! THX!
     
  2. jcsd
  3. Apr 23, 2005 #2

    Andrew Mason

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    To find the work done in compressing the spring, think about starting from equilibrium and applying a force that is just enough to compress the spring, gradually building up to 10 N.

    If you could figure out how to do this, you would be rich. Unfortunately, energy is conserved and work cannot be less than the potential energy.

    AM
     
  4. Apr 23, 2005 #3

    HallsofIvy

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  5. Apr 23, 2005 #4

    StatusX

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    There are two possibilities:

    1. The force wasn't always ten newtons, it was just enough overcome the spring force and get it compressed by 1 m (eg, if someone pushed the spring down with their hand). In this case, the force varies with distance and you would need to integrate over it to get the work done, which would come out to be 5 J (what is a C?)

    2. The force really is always ten newtons (eg, a 1 kg object under gravity), and the spring starts uncompressed with the mass is at rest. Then while the compression is less than 1 m, the mass on the spring is accelerating because there is a net force on it. Eventually it will reach a point where the force of the spring equals ten newtons, but it will still have a velocity. It will then go some extra distance until all this kinetic energy is stored in the spring (after which it would bounce back up and go into simple harmonic motion). If this is what they mean by max compression, the value of k will be greater than ten, and again, energy conservation will work out.
     
    Last edited: Apr 23, 2005
  6. Apr 23, 2005 #5
    alcatras,

    if you want to learn more on work and energy, check out this thread :
    https://www.physicsforums.com/showthread.php?t=72040


    marlon
     
  7. Apr 23, 2005 #6

    arildno

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    Eeh, it could also have been meant that the student should understand that half the work from the constant, applied force was lost (from the pool of mechanical energy), for example through heating up the spring.

    If this were the case, I think the exercise is dumb at the outset.
    It does not seem probable to me at least that the mechanical energy loss through heating is comparable to the gain in potential energy.
     
    Last edited: Apr 23, 2005
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