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Homework Help: Work done

  1. Feb 17, 2009 #1
    1. The problem statement, all variables and given/known data
    A 70kg mass is in a space and moving at 2.5 m/s. You push it with 200N for 4 m. You then move to the front of the mass and push it backwards with 170N for 3 m.

    What was the total work done?
    What was the final velocity?


    2. Relevant equations
    W = rest energy + F1*r - F2*r


    3. The attempt at a solution
    rest energy = 1/2*70*(2.5)2 = 218.75
    F1*r = 200*4 = 800
    F2*r = 170*3 = 510
    W = 218.75 + 800 - 510 = 508.75 J

    How would i get the final velocity?
     
  2. jcsd
  3. Feb 17, 2009 #2

    LowlyPion

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    Initial KE + Work = Final KE

    Initial ½mv² + W = Final ½mv²
     
  4. Feb 17, 2009 #3
    so its 218.75 + 800 + 510?
     
  5. Feb 17, 2009 #4

    LowlyPion

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    Perhaps I should rephrase that since you want to plug in numbers literally without thinking about the signs:

    Initial ½mv² + ΔW = Final ½mv²

    Btw your initial calculation incorrectly includes the initial KE in the ΔW they asked you to find.
     
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