Co-efficient of Friction (Limiting Friction)

[FeDeX_LaTeX]

Homework Statement

"A box of mass 40kg is pulled by a rope inclined at 33° to the horizontal. The box is about to slide and the tension in the rope is 190N. What is the co-efficient of friction (2 d.p.)?"

Homework Equations

F_max = uR
g = 9.8 ms^-2

The Attempt at a Solution

First, a diagram of the problem;

[PLAIN]http://img215.imageshack.us/img215/6131/m1stuck2.png

To find R, resolve upwards;

R + 190sin33° = 40*9.8
R = 392 - 190sin33°

Resolving horizontally to find F_max;

F_max = 190cos33°

So, the co-efficient of friction should be F_max divided by R;

(190cos33°)/(392 - 190sin33°) = 0.552295128(...) = 0.55 (2 d.p.)

That is my total working for the problem, but the website I am using "MyMaths" is telling me it is incorrect, and I am assuming I must be incorrect because every time a new randomly-generated problem is created it tells me I am wrong when I solve that one using the method which I have described.

Could anyone please tell me the error in my method?

Thanks.

 

[Berko]

I also get .55