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Solving Work and Force of Wagon and Child

  1. May 23, 2013 #1
    A child pulls a wagon with a force of 100N on a rope that makes a sixty degree angle with a horizontal floor. He pulls the wagon a distance of 25 meters along the floor. A: how much force is done in the direction that the wagon moves? B: How much work does the child do?


    W = FD
    W = FDCosθ



    A: W = FD

    = (100)(25)
    = 2500 Joules done in the direction that the wagon moves

    B: W = FDCosθ

    = (100)(25)Cos60
    = 1250 Joules

    Did I setup and solve these problems correctly?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 23, 2013 #2

    CAF123

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    B is right, but A is not. A is looking for a force but you found work.
     
  4. May 23, 2013 #3
    Is this the force that you are supposed to use for B or are you supposed to use the force that you are supposed to solve for?
     
  5. May 23, 2013 #4

    CAF123

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    Yes

    ??
     
  6. May 23, 2013 #5
    Is there a separate equation I need to solve for the force in part A ? I assume the force they are asking for isn't given in the information. Referring to my question above should the 100 N be used for part B or should for force found in part A be used?
     
  7. May 23, 2013 #6

    CAF123

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    Only force in direction of displacement contributes work. The force applied is at an angle to the horizontal so for part A, you need to find the component of this force in the direction of motion.
    You computed B correctly, which means you effectively already have the answer.
     
  8. May 23, 2013 #7
    I thought I understood your answer but I do not think I did. I used W=FD 1250=F(25)
    F=50N

    That does not seem correct
     
  9. May 23, 2013 #8

    Doc Al

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    Staff: Mentor

    Think of your formula for work as W = (Fcosθ)*D

    What does Fcosθ represent?
     
  10. May 23, 2013 #9
    The force in the direction it is being pulled? Which is equal to 50 Joules
     
  11. May 23, 2013 #10

    Doc Al

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    Staff: Mentor

    Right. Which is what part A asks for.

    Newtons, not Joules. (Newton the unit of force; Joule the unit of energy.)
     
  12. May 23, 2013 #11
    I guess I solved wrong but got the correct answer? Because I did W = F * Cos60

    W = 100 * Cos60
    W = 50 joules

    I appreciate your patience
     
  13. May 24, 2013 #12

    Doc Al

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    Staff: Mentor

    W = F*Cos60*25, not F*Cos60. (Work has units of force*distance, which are Joules; Force has units of Newtons.)
     
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