How Is Friction Force Calculated on an Incline?

[lu6cifer]

1.Quick question: a 12kg mass is on a 30 degree incline where the coefficient of static friction is 0.8. What is the friction force exerted on the mass?



2. F = ma, f = uN



3. I got around 83 N for this. All I did was multiply the coefficient of friction by normal force, which was Wcos30 in this case. So, 0.8 * 12 * 9.81 * cos30, giving me around 83 N, but my answer sheet says 60 N...

 

[PhanthomJay]

Remember, the static friction force is equal to uN only when the object is about to slide relative to the surface. Is it? What is the component of the gravity force acting on the object down the plane?