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Dot Product stuff ;-(

  1. Sep 16, 2013 #1
    1. The problem statement, all variables and given/known data
    A constant force of 1i - 5j -8k moves (1,-4,2) (-3,2,-1), what is the work done on the particle?


    2. Relevant equations
    Avector*Bvector=ABsinθ
    ?? I think?

    3. The attempt at a solution
    I really am quite lost... but I found the coordinates for the position vector...

    (-3-1)i + (2--4j) + (-1-2)k and the magnitude of the position vector is 7.81
    so the unit vector is -.512i + .768j -.384k... am I on the right track here?
     
  2. jcsd
  3. Sep 16, 2013 #2

    Dick

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    For a constant force F, the work done is just the dot product of the force with the displacement. Use that.
     
  4. Sep 16, 2013 #3
    so the force is 1i - 5j -8k and the displacement is -4i +6j -3k? so dot product would be -4 -30 - 24 = -58?
     
  5. Sep 16, 2013 #4
    How do I find the angel between these vectors?
     
  6. Sep 16, 2013 #5

    Dick

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    Yes, if the force isn't constant you need to work harder and integrate, but if it's constant, it's that easy.
     
  7. Sep 16, 2013 #6

    Dick

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    Why do you think you need the angle?
     
  8. Sep 17, 2013 #7

    HallsofIvy

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    If you are required, separately, to find the angle between vectors, the dot product can be defined as
    [tex]\vec{u}\cdot\vec{v}= |\vec{u}||\vec{v}|cos(\theta)[/tex]

    where [itex]\theta[/itex] is the angle between the vectors.
     
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