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Homework Help: Power concept related to gravity, constant speed, and air resistance

  1. Mar 19, 2012 #1
    An object weighting 100N is traveling vertically upward from the earth in the absence of air resistance at a constant velocity of 5 m/s. What is the power required to keep the object in motion?

    2. Relevant equations
    P = ΔE/t and P = Fvcosθ

    3. The attempt at a solution

    The explanation given in TPR(the princeton review book) is: since the object's velocity is upward and constant and the force necessary to propel the object is also upward and constant we may use the equation: P = Fv = 100N(5) = 500W.

    What I don't understand is this: If the velocity is constant, then the force should be zero, and that should make the Power zero too, right?
  2. jcsd
  3. Mar 20, 2012 #2


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    Welcome to PF!

    Hi Jmedz4nights! Welcome to PF! :smile:
    If the velocity is constant, then the total force (net force) is zero …

    so the total power is zero,

    but that's non-zero applied power,

    and equal and opposite non-zero gravitational power!

    The question is only asking for the applied power. :wink:
  4. Mar 20, 2012 #3
    Thank you tiny-tim! Where in the question stem do you infer applied power, though?
  5. Mar 20, 2012 #4


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    because it says …
    … and "power required" obviously means the power we have to add :wink:
  6. Mar 20, 2012 #5
    Thank you again all your help, tiny-tim.