# Homework Help: Coefficent of friction for a box moving at cosntant speed

1. Jul 7, 2012

### jwxie

1. The problem statement, all variables and given/known data
A horizontal force fo 190N is needed to pull a 70kg box across a horizontal floor at a constant speed. What is the coefficent of friction between the box and the floor?

2. Relevant equations

Fnet = ma

3. The attempt at a solution

My setup is F - F_f = m*a

But I have trouble getting the acceleration.....
F_f is supposed to be F_f = N * u

I am guessing F_f is knifectic fricition? Is it 190N?

2. Jul 7, 2012

### cepheid

Staff Emeritus
Yes, Ff is the force of kinetic friction. Notice that the problem says that the box moves at a constant speed. So, what is the acceleration, then? Based on that, and Newton's Second Law (which you've written above), what must be true about how Ff compares to F (the applied force)?

3. Jul 7, 2012

### jwxie

a = 0? so it means they are the same force??

So it means all we need is to use the Ff equation now.

4. Jul 7, 2012

### cepheid

Staff Emeritus
Yes, a = 0, so the forces must be the same in magnitude. (They have opposite directions, so they cancel each other out).

Yeah, so since you know the value of Ff, and you know N, you can solve for μ