How can I draw a free-body diagram for a simple object with two forces?

Start by deciding exactly which object you are analyzing. Draw that object by itself as a dot or a small box, then ask: “Which external forces act on this object in the situation I’m modeling?” A free-body diagram shows forces acting on the chosen object. Draw and label an arrow for each force included in the model. [S1]

For a two-force problem, show the two forces specified for that object. Don’t omit another force if the problem says it is present. Leave out velocity, acceleration, net force, and forces the object exerts on its surroundings; these are not additional forces acting on the isolated object in this diagram. [S1]

Suppose, purely as a diagramming example, a problem explicitly states that a chosen object has two external forces: force A to the right and force B upward. Those directions and names are assumptions for this example, not a claim about a particular object or a complete physical situation.

          ↑ B
          |
          • ───→ A
       chosen object

The dot represents the isolated object; label each arrow with the force’s name or symbol. You could use descriptive labels such as “push” if the problem provides that name. Don’t add an arrow just because a force seems likely in some real-world setting: include it if it acts on the chosen object in the stated model. If the problem specifies another force, represent it too. [S1]

Use an “on the object” test for every arrow: can you say that this force acts on the object you isolated? If the force is instead exerted by that object on something else, it belongs on a diagram for that other object. For multiple objects, draw a separate diagram for each. [S1]

Then check what each arrow represents. Keep arrows for external forces on the chosen object; don’t add arrows for motion, acceleration, or net force. If the question’s description is unclear about which forces are included, note the ambiguity rather than silently treating your sketch as complete. [S1]

Write the selected object beside the sketch, then draw a dot or box and add only the external force arrows specified for it. A two-arrow layout is appropriate only when the model really contains two forces acting on that object. The directions below are placeholders, not defaults:

Object: [chosen object]

          ↑ [Force 2: name or symbol]
          |
          • ───→ [Force 1: name or symbol]

Match the arrow directions to the problem. Before using the diagram, check that each arrow represents a force on the selected object and that you have not included motion, acceleration, net force, or a force exerted by the object on something else. The diagram organizes the stated model; it does not establish that the description is complete. [S1]