# Homework Help: Stoke's law and its applications

1. Sep 18, 2011

### logearav

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

Stoke's law states that retarding force is proportional to velocity. An example is falling rain drop in air.
My book states that " if a rain drop falls, it accelerates initially due to gravity. As the velocity increases, the retarding force force also increases. Finally, when viscous force plus buoyant force becomes equal to force due to gravity, the net force becomes zero and so does the acceleration.
The rain drop then descends with a constant velocity
2. Relevant equations

3. The attempt at a solution
My question is, if acceleration is zero, how the rain drop would move with constant velocity?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 18, 2011

### Dick

Acceleration is the rate of change of velocity with time. If acceleration is zero, how can the velocity change???

3. Sep 18, 2011

### logearav

So, even if there is no acceleration, a body will move. Am I interpreted right?
Thanks for the reply Mr. Dick

4. Sep 18, 2011

### Dick

If a body is a rest and there is no acceleration, it will not start moving. If body is moving at velocity v and there is no acceleration then it will continue moving at velocity v.

5. Sep 18, 2011

### logearav

I got it now. Thanks a lot Mr. Dick