Question

A block slides down a rough ramp with a 30-degree incline as shown.

A diagram of a block on a 30 degree incline.

Which diagram represents the forces acting on the block?

A free body diagram with three forces: a force vector northeast labeled F Subscript N Baseline, a force vector southeast labeled F Subscript f Baseline, and a force vector south labeled F Subscript g Baseline.

A free body diagram with three forces: a force vector northeast labeled F Subscript N Baseline, a force vector southeast labeled F Subscript f Baseline, and a force vector southwest labeled F Subscript g Baseline that is equal in magnitude but opposite in direction to the vector pointing northeast.

A free body diagram with three forces: a force vector north labeled F Subscript N Baseline, a force vector southeast labeled F Subscript f Baseline, and a force vector south labeled F Subscript g Baseline, which is equal in magnitude but opposite in direction the north vector.

A free body diagram with three forces: a force vector northeast labeled F Subscript N Baseline, a force vector northwest labeled F Subscript f Baseline, and a force vector south labeled F Subscript g Baseline.

Answer 1

The fourth diagram.

Reason: gravity (Fg) is vertically downward. The normal force (FN) is perpendicular to the plane (northeast in the diagram), and kinetic friction (Ff) acts parallel to the plane opposite the motion — the block is sliding down so friction points up the plane (northwest). The other choices place friction or the normal in incorrect directions (or make FN oppose gravity, which is not true).