# Homework Help: Viscosity,Fluid flow and terimal velocity

1. Dec 1, 2011

### ChunkymonkeyI

1. The problem statement, all variables and given/known data
When does Fb(Buoyancy force) equal the drag force(FD)?
and Fb+FD=Fg

2. Relevant equations
Fb=density of water times volume of water times 9.80 m/s^2
FD=6pi times viscosity times radius times v
Fg=mg
3. The attempt at a solution
All ik is the solid has 2 be in the air going to the water but idk how 2 really prove that please help me

2. Dec 2, 2011

### RTW69

This problem is really confusing. Do we have a ball falling in water? Not sure what is going on here.

3. Dec 2, 2011

### ehild

It is a sphere of radius r moving with speed v in a fluid of viscosity η. The drag force is FD=6πηrv.
The buoyant force is FB=ρVg, where ρ is the density of the liquid and V is the volume of the sphere.

ehild

PS: ChunkymonkeyI, what did you mean: "All ik is the solid has 2 be in the air going to the water but idk how 2 really prove that please help me" We use English here.

Last edited: Dec 2, 2011