# Coefficent of friction for a box moving at cosntant speed

## Homework Statement

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?

Fnet = ma

## 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?

cepheid
Staff Emeritus
Gold Member

## Homework Statement

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?

Fnet = ma

## 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?

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)?

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

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

cepheid
Staff Emeritus