Where do you put the angle in force diagrams?

[ewolf335]

Homework Statement

Ok, so I'm in college now. However, I'm having the same problem i did when I was in high school. When your doing a problem that has let's say a truck on a 30 degree incline, how do you decide whether that angle will be touching the y component of the angle or the x? The reason I ask is because I was told the component touching the angle is always the x component, but in some problems that doesn't appear to be true, so I don't know how to draw my own force diagrams without mixing up the two.

Homework Equations

equation for force of kinetic friction

The Attempt at a Solution

 

[mizzy]

if the question says that the incline makes an angle with the horizontal, then that horizontal is the x component.