Forces on an Incline

On a ramp an object feels three forces: its weight $\relax mg$ straight down, the normal force $N$ perpendicular to the ramp, and friction $f$ along the surface, against the sliding.

Three forces on a box resting on a ramp: mg straight down, N perpendicular to the surface, f along it.

Choose the x axis along the slope and the y axis perpendicular to it. Only the weight is slanted, and nothing moves into the slope:

$$\sum F_{\parallel} = m a, \quad \sum F_{\perp} = 0$$