Power concept related to gravity, constant speed, and air resistance

  • #1

Homework Statement


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?


Homework Equations


P = ΔE/t and P = Fvcosθ


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?
 

Answers and Replies

  • #2
tiny-tim
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Homework Helper
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Welcome to PF!

Hi Jmedz4nights! Welcome to PF! :smile:
If the velocity is constant, then the force should be zero, and that should make the Power zero too, right?

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:
 
  • #3
Thank you tiny-tim! Where in the question stem do you infer applied power, though?
 
  • #4
tiny-tim
Science Advisor
Homework Helper
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because it says …
What is the power required to keep the object in motion?

… and "power required" obviously means the power we have to add :wink:
 
  • #5
Thank you again all your help, tiny-tim.
 

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