Work done by Friction on an Inclined Plane

[fajoler]

Homework Statement

A luggage handler pulls a suitcase of mass 17.1 kg up a ramp inclined at an angle 24.0 above the horizontal by a force of magnitude 146 N that acts parallel to the ramp. The coefficient of kinetic friction between the ramp and the incline is 0.257. The suitcase travels a distance 3.90 m along the ramp.

What is the work done on the suitcase by the friction force?

Homework Equations

W = F o r = |F|*|r|*cos(theta)
Friction Force = uN

The Attempt at a Solution

Ok so I calculated that the Friction Force should equal umg since the direction of movement is along the incline rather than in the x direction. So Friction Force = 43.06806 N. If we multiply it by the change in distance, the Work done by friction should be -168 J, but this doesn't seem to be the correct answer. Anyone know where I may have screwed up?

 

[issacnewton]

well force of friction is not $$\mu mg$$ . set up a free body diagram first.

 

[fajoler]

Oh wow... I'm sorry that was a stupid move on my part. For some reason I forgot that the normal force depends on gravity AND the direction of the gravitational force... Thanks so much I appreciate it!