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Homework Help: Change in the object's kinetic energy

  1. Dec 14, 2004 #1
    A force acting on an object given by the function F(x)=3x^2 + 4, where F is in Newtons and x is in meters. what is the change in the object's kinetic energy as it mores from x=1 to x=3 m?

    W=6x
    it changes from 6 J to 18 J
    the answer 12 J is not the correct one
    it says 26 J

    why?
     
  2. jcsd
  3. Dec 14, 2004 #2
    Why is W = 6x?
     
  4. Dec 14, 2004 #3
    W=F’=∫P

    F(x)=3x^2 + 4
    F'(x)=6x
     
  5. Dec 14, 2004 #4

    Pyrrhus

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    This is what i get

    [tex] W = \int^{3}_{1} (3x^2 + 4)\vec{i} \cdot dx \vec{i} [/tex]

    [tex] W = x^3 + 4x]^{3}_{1} = 34 J [/tex]

    Work-Kinetic Energy Principle

    [tex] \sum_{i=1}^{n} W_{i} = \Delta K [/tex]

    so

    [tex] \Delta K = 34J [/tex]
     
  6. Dec 14, 2004 #5
    that's not even one of the choices. Does it matter if it was F(x) instead of F(t)?
     
  7. Dec 14, 2004 #6
    Just so you know, i believe that 26 is wrong also.

    EDIT*Looks like cyclovenom got there first. The 34 is correct.

    Remember, W is not F', its the other way around.

    F = W'


    And yes it does matter if it was F(x) instead of F(t).
     
    Last edited: Dec 14, 2004
  8. Dec 14, 2004 #7
    how would F be different if it was F(t)?
     
  9. Dec 14, 2004 #8

    Pyrrhus

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    I am with Parth Dave, are you sure about this, urban?
     
  10. Dec 14, 2004 #9
    the packet must have a typo then

    Cyclovenom, I never see you on ICQ :)
     
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