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Work Done by a Constant Force (Q6 page 188 of Cutnell and Johnson)

  1. Aug 15, 2014 #1
    6.A person pulls a toboggan for a distance of 35.0 m along the snow with a rope directed 25.0° above the snow. The tension in the rope is 94.0 N.

    a) How much work is done on the toboggan by the tension force?


    Ok, so W = (F cos Θ) s
    = (94.0 cos 25°) 25 m
    = WRONG ANSWER

    I know the answer is 94.0 times distance but that would equal WORK right (fs)

    why are they doing (94.0 N)(35.0 M) = Force???

    **this is really frustrating , why are they including distance as force?
     
    Last edited: Aug 16, 2014
  2. jcsd
  3. Aug 15, 2014 #2

    olivermsun

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    So what does it say in terms of a "right" answer or solution?

    Why does your solution have 94.0 twice?
     
  4. Aug 16, 2014 #3

    NascentOxygen

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    Hi gcombina. What does that "s" in your formula stand for? What units would it have?

    Also, what are the units of that "W"?
     
  5. Aug 16, 2014 #4
    this is the "right" answer

    W = (94.0 N) (35.0 m) cos 25.0 = 2980 J
     
  6. Aug 16, 2014 #5

    it was my mistake, i corrected it already.
     
  7. Aug 16, 2014 #6

    basically, this is the right answer => (94.0 N) (35.0 m) cos 25° =2980 J

    this is formula (F cos θ)s I always use, but now they are using (Fs cos θ), where is this formula comes from?????

    I though W = (F cos θ)s and NOT (Fs cos θ)
     
  8. Aug 16, 2014 #7

    Orodruin

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    Uhmmmm ... It is the same. Multiplication is commutative so it does not matter what order you multiply things in.
     
  9. Aug 16, 2014 #8

    NascentOxygen

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    Do these give the same answer?
     
  10. Aug 18, 2014 #9
    no, they don't give the same answer

    Fs cos θ => (94.0 N) (35.0 m) cos 25° =2980 J

    AND

    F cos θ => (94.0 N) cos 25° = 85.19 J
     
  11. Aug 18, 2014 #10

    Nathanael

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    Check your units. You wrote Joules but it is actually Newtons.

    I think you meant "(F cos θ)s"? In which case, you do get the same answer.
     
  12. Aug 18, 2014 #11

    haruspex

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    The distance is 35m, not 25m.
     
  13. Aug 18, 2014 #12

    NascentOxygen

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    There is something very fundamental that you are misunderstanding, but I'm having difficulty identifying what.

    They aren't. It's difficult to see why you might think they are.

    F s cosθ = Force x distance x cosθ
     
  14. Aug 18, 2014 #13

    olivermsun

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    Starting from the beginning, step by step:

    W = (F cos Θ) s

    The distance is s = 35.0 m
    The force in the direction that the distance is measured over is F cos Θ = (94.0 N)(cos 25°)

    W = (F cos Θ) s = (94.0 N)(cos 25°)(35.0 m) = 2980 N m = 2980 J.
     
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