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Work on a packing crate

  1. Sep 25, 2014 #1
    1. The problem statement, all variables and given/known data
    A 51-kg packing crate is pulled with a constant speed across a rough floor with a rope that is at an angle of 43.5 degrees above the horizontal. if the tension in the rope is 115 N, how much work is done on the crate to move it 8.0 m?

    2. Relevant equations
    Do we use frictional force when calculating work?


    3. The attempt at a solution
    W=Fd where F is the force only in the x direction. W=115cos(43.5)8.0
     
  2. jcsd
  3. Sep 25, 2014 #2

    Simon Bridge

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    Welcome to PF;
    Did you have a question?

    The approach looks good to me - what you have done so far would get you 2 out of a possible 3 marks from me.
     
  4. Sep 25, 2014 #3
    Well, my specific question was if I have to factor in friction/when I have to factor in friction to work in general. Why 2 out of 3 marks? Because I didn't simplify my answer?
     
  5. Sep 25, 2014 #4

    Simon Bridge

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    You are right to wonder - the question asks for the work done on the crate, but does not tell you which force to consider.
    Since the total mechanical energy of the crate remains unchanged, the net work on the crate must be zero.
    (Well, the crate maybe got hotter.)

    You may want to make sure the crate does, indeed, stay on the floor. I didn't check.
    You have calculated the work done by the applied force - which is usually what questions written that way intend.

    I would normally give one mark at this level for the correct units on the final answer.
     
  6. Sep 26, 2014 #5
    Oh! I always forget that constant velocity means that acceleration is zero, which means there's no force. No force means that there's no net work, if I'm understanding you correctly. Thanks for clearing that up!
     
  7. Sep 26, 2014 #6

    Simon Bridge

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    That's right - you just need to use your knowledge of how your teacher sets things up to work out if the question wants the work done by the applied force or the total change in mechanical energy.
     
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