Imagine a box sliding down a rough slope. What will the free body diagram for the box look like?

In a free body diagram for a box on a rough slope, there will be three arrows. One will point out of the box up the slope (parallel to the slope), one will point directly downwards from the box, and the final one will point upwards out of the box perpendicular to the slope (diagram to be drawn on whiteboard).The downwards arrow is the weight of the box (W=Mg). This is the force which will cause the box to slide down the slope. The arrow parallel to the slope is friction due to the rough slope: friction is a force which opposes motion, and so points opposite to the direction the box slides down the slope. Finally, the arrow perpendicular to the slope is the normal reaction from the slope: there is a component of the box's weight that pushes perpendicular to the slope, so the slope "reacts" and pushes back so the box doesn't fall through the surface.