A.2.3—Free-body diagrams

Isolate one body

A free-body diagram shows only the chosen body and the external forces acting on it. Replace the body with a point or simple shape and choose useful axes.

Draw actual forces

Use arrows from the body, label each interaction and draw the direction physically. Typical labels include weight $mg$ , normal force $N$ , tension $T$ , friction and drag.

Resolve only when needed

If a force is angled, resolve it into the chosen axes. Then apply $\sum F_x=ma_x$ and $\sum F_y=ma_y$ to the same body.

Common trap

Do not draw velocity, acceleration or a force exerted by the chosen body on its surroundings as forces acting on the chosen body.

How it is assessed

The evidence asks for a labelled diagram of a ball supported by a tension, rewarding the correct force labels, directions and omission of non-forces.

Command terms

Draw

What earns marks

Choose the stated object, draw only external forces, label weight and tension/normal/contact forces, and orient them correctly. Resolve angled forces only after the free-body diagram is complete.

Watch for

Including velocity or acceleration as arrows, or drawing the reaction force on the supporting body instead of the force on the chosen ball.

Representative question

[Maximum number: 2]

Draw a labelled free-body diagram of the forces on the ball.

Labelled vertical weight / $\mathrm{W} / \mathrm{mg} / \mathrm{F}_{\mathrm{g}}$ and tension / $\mathrm{T} / \mathrm{F}_{\mathrm{T}}$ in approximately correct direction

With vertical T component equal to weight

Ignore other forces drawn for MP1

[2]

Build the force model

Choose the system, draw a labelled free-body diagram, classify the interactions and resolve components. Apply Newton’s laws with the correct boundary: contact forces, field forces, friction, tension, buoyancy and restoring forces each have their own direction and conditions.

Track momentum

Use $ec p=m ec v$ , $ec J=\Delta ec p$ and momentum conservation only after checking external impulse. Distinguish elastic and inelastic collisions, explosions and energy transfer.

Track circular motion

The inward resultant provides $a_c=v^2/r=\omega^2r$ . It may come from tension, gravity, normal, friction or a field force. Angular and linear descriptions are linked by $v=\omega r=2\pi r/T$ .

Final checks

Ask: Which body is the system? Which forces are external? Is mass constant? Is acceleration uniform or radial? Is kinetic energy conserved, transferred or increased?

A.2.3—Free-body diagrams

Draw a Labelled Free-Body Diagram

A.2.3 Exam Analysis

Assessment in practice

Retrieve the A.2 Forces and Momentum Model

Question 1