Coefficent of friction for a box moving at cosntant speed

[jwxie]

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 coefficient of friction between the box and the floor?

Homework Equations

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?

I am confused. Please help!

 

[cepheid]

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 coefficient of friction between the box and the floor?

Homework Equations

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?

I am confused. Please help!

Yes, F_f 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 F_f compares to F (the applied force)?

 

[jwxie]

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

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

 

[cepheid]

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

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

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 F_f, and you know N, you can solve for μ