# A raindrop of mass 3.35x10[SUP]-5[/SUP] falls vertically at constant speed under the

1. Oct 11, 2012

### AryRezvani

1. The problem statement, all variables and given/known data

A raindrop of mass 3.35x10-5 falls vertically at constant speed under the influence of gravity and air resistance. Model the drop as a particle. As it falls 100m, what is the work done on the raindrop. (a) by the gravitational force and (b) the air resistance.

2. Relevant equations

FΔrCosθ=W
Fg=mg

3. The attempt at a solution

Fg=mg=3.34x10-4

Force of gravity is acting downward, and air resistance is acting upwards so the net force is taken into account when using FΔrCosθ, right?

Lost after this step.

Why is the force constant? Just wondering?

2. Oct 12, 2012

### Woopydalan

Re: A raindrop of mass 3.35x10-5 falls vertically at constant speed under

weight is a constant force. Air resistance is assumed to be constant at this level as well.

3. Oct 12, 2012

### voko

Re: A raindrop of mass 3.35x10-5 falls vertically at constant speed under

The drop is falling at constant speed. So the net force is zero.

4. Oct 12, 2012

### AryRezvani

Re: A raindrop of mass 3.35x10-5 falls vertically at constant speed under

Force of gravity = 9.8
Displacement = 100
Cos (90) = 1

But if the net force is zero, what do we plug into F for the equation?

5. Oct 12, 2012

### PhanthomJay

Re: A raindrop of mass 3.35x10-5 falls vertically at constant speed under

The problem is asking you to find
a.) the work done by gravity and
b) the work done by the air resistance.
If the net force is 0 , which you should first ask yourself whether or why this is true, that would be key to finding the air resistance force, and the work done by it.

6. Oct 12, 2012

### nasu

Re: A raindrop of mass 3.35x10-5 falls vertically at constant speed under

Force of gravity is not 9.8. Check your values.
The questions tells you what force to plug into equation, for each part (a - force of gravity, b - force of air resistance).
The condition net force= 0 allows you to calculate the force of air resistance.