Coefficent of friction for a box moving at cosntant speed

  • Thread starter jwxie
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  • #1
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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?


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!
 

Answers and Replies

  • #2
cepheid
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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?


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

So it means all we need is to use the Ff equation now.
 
  • #4
cepheid
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Science Advisor
Gold Member
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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 Ff, and you know N, you can solve for μ
 

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