When drawing a free body diagram:

Question

BlackTom AI · Accepted Answer

Question restatement: The prompt asks about best practices when drawing a free body diagram (FBD) and provides four statements to evaluate.
Option A: 'we should include the acceleration.' In a free body diagram, you depict forces, not the motion quantity itself. Acceleration is a result you solve for using the net force, mass, and Newton's second law, but it is not drawn as a vector on the FBD. So including acceleration on the diagram would mix kinematic information with force representation.
Option B: 'we must include all objects in the same diagram.' Free body diagrams focus on a single object of interest and the forces acting on that object. Other objects are represented only insofar as they exert forces on the object of interest (e.g., a support, a person applying a push). Including every object in the system would clutter the diagram and obscure the forces on the chosen body.
Option C: 'we should include only external forces acting on the object, with no reaction forces.' This aligns with the standard definition of a free body diagram: it shows all forces exerted on the object by external sources, such as gravity, normal force, friction, tension, and applied forces. Reaction forces to those interactions (the action-reaction pairs) act on other bodies, not on the object in question, so they are not drawn on its FBD. This option captures the correct scope of an FBD.
Option D: 'forces should be drawn as arrows pointing towards the free body.' In a free body diagram, forces are drawn as arrows whose tails are at the point where the force is applied and whose directions reflect the force direction on the object. The arrow should indicate the direction of the force on the object, not necessarily always toward the free body itself. Saying arrows point toward the free body is misleading; the correct convention is that arrow directions represent the force acting on the object, emanating from the point of contact or action, not pointing "toward" the body as a general rule.
Each option highlights a common point of confusion: combining kinematics with force (A), modeling multiple objects in one FBD (B), including action-reaction pairs on the same body (C vs. D). Distinguishing external forces from internal or reaction forces is essential for accurate free body diagrams, and the arrow convention must reflect the force acting on the body.

PHYS1001 (ND) Topic 2 Pre-work Quiz

When drawing a free body diagram:

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