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

1. Mar 19, 2012

### Jmedz4nights

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. Mar 20, 2012

### tiny-tim

Welcome to PF!

Hi Jmedz4nights! Welcome to PF!
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.

3. Mar 20, 2012

### Jmedz4nights

Thank you tiny-tim! Where in the question stem do you infer applied power, though?

4. Mar 20, 2012

### tiny-tim

because it says …
… and "power required" obviously means the power we have to add

5. Mar 20, 2012

### Jmedz4nights

Thank you again all your help, tiny-tim.