1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Mar 19, 2012 #1
    1. The problem statement, all variables and given/known data
    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


    User Avatar
    Science Advisor
    Homework Helper

    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


    User Avatar
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook