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Work and force problem with vectors

  1. Jul 1, 2015 #1
    1. The problem statement, all variables and given/known data
    A machine carries a 4.0 kg package from an initial position of di=0.6m##\hat i## + 0.7m##\hat j## + 0.26m##\hat k## at t = 0 to a final position of df= 11.5m##\hat i## + 14.0m##\hat j## + 10.2m##\hat k## at t = 15.0 s. The constant force applied by the machine on the package is F= 10.9N##\hat i## + 13.25N##\hat j## + 9.94N##\hat k##. For that displacement, find (a) the work done on the package by the machine's force and (b) the average power of the machine's force on the package.

    2. Relevant equations
    w=Fd

    3. The attempt at a solution
    im attempting to use the dot product here to get work but not working, i dont know why

    20150701_191726_zpsbylgqsfn.jpg
     
  2. jcsd
  3. Jul 1, 2015 #2
    Quick question: should you have a vector as the result of the dot product?
     
  4. Jul 1, 2015 #3
    never
     
  5. Jul 1, 2015 #4
    thanks, i have been doing that lately i dont know why, i must stop doing that with the dot product.
     
  6. Jul 1, 2015 #5

    Nathanael

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    Homework Helper

    Is the j component of the initial position 0.7m or 0.75m? (You said 0.7 in the OP but I think it's a typo.)

    Also what is the force supposed to be? Your OP says <10.9, 13.25, 9.94>N but your work treats the force as <5, 5, 7>N
     
  7. Jul 1, 2015 #6
    yes your right its 0.75j for initial pos.
    the force is supposed to be (5,5,7)

    i made a few typos on the OP sorry Nathanael.
     
  8. Jul 1, 2015 #7

    Nathanael

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    Homework Helper

    Ok, your only mistake is miscalculating the dot product. ##\vec a\cdot \vec b=a_xb_x+a_yb_y+a_zb_z##
     
  9. Jul 2, 2015 #8
    Ha ha.. We all do stuff like that sometimes, and it's much better to do that here than on the exam!
     
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