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Homework Help: Problems with Power only using velocity and force

  1. Sep 14, 2015 #1
    1. The problem statement, all variables and given/known data
    An object moving with a velocity whose components are [4m/s,-1m/s,3m/s] is acted on by a force whose components are [-5N,0,+5N]. What is the power of the energy transfer involved in this interaction?

    2. Relevant equations

    3. The attempt at a solution
    4m/s*-5N= -20W

    (202+152+02)sqrt=Magnitude = 25W which could be the answer but I'm not sure. I just dont know how negative velocity affects power because P=Fv so if F is 5N and v=-3m/s then it would seem like -15W is the answer but im not sure. I also don't really have experience with 3-component vectors.
  2. jcsd
  3. Sep 14, 2015 #2


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    Science Advisor
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    Gold Member

    It's OK up to here.

    This treats the three amounts you have calculated as vectors, but they are not. Power is not a vector. So you don't square and add to get a magnitude. You just add.

    Power is the scalar product (dot product) of force and velocity.
  4. Sep 14, 2015 #3
    Thank you for your reply, that does make sense that Power isn't applied in a direction. So the answer is -5W
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