Drawing Free-Body Diagrams

A free-body diagram strips away the clutter of ropes, surfaces, and other objects so you can see exactly what pushes or pulls on the one object you care about. Every rope becomes a tension arrow, every surface becomes a normal force arrow (and a friction arrow if the surface is rough), and gravity becomes a single arrow pointing straight down from the center of the object, drawn as a dot. The object's own size and shape stop mattering: all that matters is which forces act on it and in which directions.

Deciding what belongs on the diagram comes down to one test: is something physically touching the object or exerting a field on it? Tension, normal force, friction, applied pushes, and gravity all pass that test. Quantities like velocity, acceleration, momentum, and centripetal force do not belong, because they describe motion or describe the net effect of forces, not a force in themselves. Centripetal force is just the label for whatever net force happens to point toward the center of a circular path; if a string supplies that net force, you draw the tension arrow and stop there.

Once the diagram is drawn, you pick axes, split any tilted forces into components, and write Newton's second law separately for each direction. That step is where this topic connects to what comes next: equilibrium problems set the sum of forces to zero in every direction, while accelerating situations, like a box in a speeding-up elevator, set the net force along the direction of acceleration equal to mass times that acceleration instead of zero. The diagram itself only shows the forces present at that instant; how those forces must add up depends entirely on what the object is doing.