Dynamics: (e) free-body diagrams

The idea

A free-body diagram shows a single object isolated from its surroundings, with all external forces acting on it represented as arrows. Each arrow's length indicates magnitude and its direction shows the force's direction. The object is drawn as a simple shape or dot. Forces to include: weight, normal contact force, tension, friction, upthrust, applied forces. Do not include forces the object exerts on other things. For example, a block on a rough slope has weight downwards, normal contact force perpendicular to the slope, and friction along the slope. In MCQs, you may select the correct diagram or identify a missing force. Ensure arrows start from the object and are labelled.

What earns the marks

• A free-body diagram shows one object isolated with all external forces acting on it.
• Forces are represented by arrows; length indicates magnitude, direction indicates direction.
• Only forces acting on the object are included, not forces it exerts on other objects.
• Common forces: weight, normal contact force, tension, friction, upthrust, applied force.
• The diagram helps analyse equilibrium or acceleration using Newton's laws.

Common mistakes and how to put them right

• Including forces that the object exerts on other objects.

  Put it right by showing only forces acting on the chosen object.

• Drawing weight as a line from the centre but forgetting to label it.

  Put it right by labelling each arrow with the type of force.

• Omitting the normal contact force when an object rests on a surface.

  Put it right by including a force perpendicular to the surface.

• Drawing arrows with incorrect relative lengths when magnitudes are known.

  Put it right by scaling arrow lengths to represent relative magnitudes.

Exam tips

• Draw the object as a dot or simple box and draw all forces starting from it.
• Label each arrow clearly (e.g. W, N, T, F) and ensure directions are correct.
• Check that you have not included any internal forces or forces on other objects.