# Help understanding the Viscosity Formula

1. Sep 17, 2011

### cpscdave

1.

Hi there,
We have some questions regarding the force it would take to keep a shaft moving in a tube at a constant velocity.

I gather the formula we should use is:

$$F/A=u*dux/dy$$

My understanding is that in this formula
$$F$$ is the force required
$$A$$ is the parallel surface area
$$u$$ is the viscosity of the fluid
and
$$dux/dy$$ is the slope of the velocity gradient.
Since in the question the shaft is up to speed $$dux/dy=1$$
I punch in the numbers and the result I'm getting have been nonsensical so any clarification would be fantastic!!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution