# Homework Help: Resistive force

1. Feb 4, 2004

### anneseanandy

I need to calculate the force required to pull a copper ball radius 2.00cm upward through a fluid at the constant speed 9.00cm/s. The drag forse is to be proportional to the speed, with proportionality constant .950kg/s. Ignore any boyant force.

What I did so far was figure out the resistive force using the equation: R=-bv, where b= .950kg/s and v= .09m/s
So that gives me -.0855Newtons.

My equation set up so far is:
SumYForces = mg+ Resitive force= Pull Force
and then from there I have no clue, the answer in the back of the book is 3.01 N, but I just can't seem to get it. Thanks -anne

2. Feb 4, 2004

### gnome

How about the weight of the ball?

PS: the unit of that proportionality constant is not kg/s. It must be N-s/m so that when you multiply it by m/s you end up with N.

Last edited: Feb 4, 2004
3. Feb 4, 2004

### anneseanandy

the weird thing is they give no mass, and thats what they give as the proportional constant.

4. Feb 4, 2004

### gnome

You are given the radius of the ball. From that, with a little effort, you can find the mass (and then the weight).

As to the unit of the constant, you're right. Sorry, I guess I was too tired last night. (1kg/s)*(1m/s)=1(kg-m/s^2)=1N