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Calculating work done

  1. Aug 27, 2014 #1
    1. The problem statement, all variables and given/known data

    A carpenter is holding two tool boxes of mass 19kg and 21kg respectively. Calculate the work done to carry the tool boxes through a horizontal distance of 16m.

    2. Relevant equations

    W = mgh

    3. The attempt at a solution

    m = 21kg + 19kg
    g = 9.81ms^-2
    h = 0

    W = (21 + 19)(9.81)(0)
    W = 0J

    Is this correct? And if this is correct, would you explain why it is 0J?
     
  2. jcsd
  3. Aug 27, 2014 #2

    NascentOxygen

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    Staff: Mentor

    The formula for work involves the dot product of two vectors.

    W = F . s

    Does this give you a hint?

    (Yes, your answer of 0J is correct.)
     
    Last edited: Aug 27, 2014
  4. Aug 27, 2014 #3

    HallsofIvy

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    Staff Emeritus
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    In order to do work, you must move an object with mass in the direction of a force vector.
     
  5. Aug 27, 2014 #4
    So if I use W = F . s, then it would be

    W = (19 + 21)(9.81)(16) ?
    Or is it,
    W = (19 + 21)(9.81)(16) cos 90 ?
     
  6. Aug 27, 2014 #5

    NascentOxygen

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    Staff: Mentor

    this ^^^

    Instead of writing the vector formula, it is often expressed as W = F s cosθ
    and that's probably the best one to memorise.
     
  7. Aug 27, 2014 #6
    NascentOxygen, would you tell me why the angle is 90? Shouldnt it be 0? And how about if I use sin 0? I will still get 0J.
     
  8. Aug 27, 2014 #7

    NascentOxygen

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    The question just tests how easily confused you are.

    The only force mentioned is weight, and it acts downwards ⇓
    while the motion described is horizontal ⇒
    and the angle between those two vector quantities is ..... ?

    You want to invent your own formula using sin 0° ? What angle in this picture is 0°?
     
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