What is the coefficient of kinetic friction

1. ilkjester

35
1. The problem statement, all variables and given/known data
If you use a horizontal force of 30.0 N to slide a 12.0-kg wooden crate across a floor at a constant velocity, what is the coefficient of kinetic friction between the crate and the floor.

2. Relevant equations
Fkf=μFn

3. The attempt at a solution
Im not sure how to find the μ

Last edited by a moderator: Mar 7, 2013
2. PhanthomJay

6,333
You need to identify all the forces acting on the crate, and then apply Newton's laws in both the x and y direction.

3. ilkjester

35
Humm I dont think i know how to do it still but i dont know ill try my attempt at the problem and post what i get.

4. ilkjester

35
Ok well i got 117.6 when i did the problem not sure if its right or not.

5. PhanthomJay

6,333
You got 117.6 for which force? Please show your work.

6. ilkjester

35
Well i have no idea how i did it or if i even did it right. But this is what i did. I kinda just copied a similar problem they did in the book and filled in the differences.
Fp= μmg
fp= μ(12kg(9.8)
fp-117.6

7. pooface

210
no 117.72 N is the Normal force on the crate.

8. ilkjester

35
aww man and i thought i had something. Well I guess ill have to keep trying something I dont know why im not getting this problem the rest of them were easy i think once i get it im gonna be like duhh.

9. sdstewar

2
Think about newtons second law. The object is in constant velocity, so there is no NET force on the object. It's in equilibrium.

10. ilkjester

35
ok i think ive got this now. But its assuming that the floor is wooden.
Kinetic friction force Ff kinetic =μkFn
Now μk wood on wood equals 0.20
so Ff kinetic=0.20(30)
Ff kinetic=6
does this sound right.

11. sdstewar

2
Well.. the question says your trying to find the coefficient of kinetic friction, not the force of kinetic friction.

12. ilkjester

35
Man looks like its back to the drawing boards again. Thank you all for the help and tips so far though.

13. Sourabh N

633
You can find Fn by balancing forces in vertical direction. Now frictional force is balanced by external force (30 N). Thus you can find $$\mu$$$$_{k}$$

Cheers

Last edited: Oct 9, 2007
14. ilkjester

35
Ok now i think i might have something or its just my bizarre attempt at something.
The coefficient of kinetic friction is defined as the ratio of the kinetic friction force (F) between the surfaces in contact to the normal force: Ff/N.
So Ff/N which i found out earlier the friction force is 6 and the normal force is 117.72 so 6/117.72=.0509

15. ilkjester

35
so would it be 6=μk(30N)
then divide by 30 on both sides so 6/30=.2μk

Staff: Mentor

You are supposed to compute $u_k$, not look it up. And no, this is not correct. The crate will accelerate at 24N/12kg=2 m/s2 if a person applies a 30N horizontal force in one direction and friction only applies a 6N force in the opposite direction.

You are making this too hard. The crate is moving at a constant velocity. This means that the net horizontal force acting on the crate is zero. The person is applying a 30N force in one direction. To make the net horizontal force zero, friction must therefore be applying a 30N force in the opposite direction. The coefficient of kinetic friction is just the ratio of the frictional force (which you now know) to the normal force (which you calculated in post #4).

17. ilkjester

35
so was i right in my one post then

Ok now i think i might have something or its just my bizarre attempt at something.
The coefficient of kinetic friction is defined as the ratio of the kinetic friction force (F) between the surfaces in contact to the normal force: Ff/N.
So Ff/N which i found out earlier the friction force is 6 and the normal force is 117.72 so 6/117.72=.0509

18. Sourabh N

633
sorry to tell you but frictional force is 30 N, exactly equal to the horizontal force applied (since the object is moving with constant velocity) and normal force is exactly equal to gravitational force (since object is not moving in vertical direction). Their ratio is what you need.

Staff: Mentor

How did you get that out of what I wrote?? The frictional force is 30N.

20. ilkjester

35
I have no idea but thank you for all the help.

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