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Free-body diagrams

Key idea: Balanced and unbalanced forces, two-dimensional free-body diagrams, F = ma, friction and action–reaction pairs.

  • SEC G3 Combined Science Physics component 2027
Syllabus and review details

Free-body diagrams may involve forces in at most two dimensions. This Science lesson excludes graphical three-force equilibrium, inertia as a separate extension and terminal-velocity treatment.

Retrieval check
Before starting, name the body you are analysing and distinguish velocity from acceleration. The answer depends on the selected body and the change in velocity, not on motion alone.

Build the force model

Free-body diagram for a crate pulled across a rough floorA crate is isolated as a box. Four arrows start at it: normal force upward, weight downward, pull to the right and friction to the left. A separate dashed arrow to the right is labelled motion, not force.Forces on the crate onlynormal forceweightpullfrictionmotion (not a force)vertical resultant = 0horizontal resultant →
Isolate the named object and draw only external forces acting on it. The dashed motion arrow is contextual information, not another force. Analyse horizontal and vertical resultants separately.

A free-body diagram isolates one body and shows every external force acting on it as a labelled vector. Begin from interactions, not from a memorised list of arrows.

Worked reasoning

Modelled example 1

Model the decisive reasoning

Core

Problem

A crate is pulled right across a rough horizontal floor. Construct the free-body model for the crate.
Study the worked solution

  1. Select and apply the model

    Method

    Isolate the crate and identify its four external interactions.

    Reason

    A free-body diagram contains only forces acting on the selected body.

    Working

    Weight down; normal force up; applied pull right; friction left.

Guided practice 2

Complete a guided force model

About 5 min

Problem

A lamp hangs from one vertical cable while a steady wind pushes it right. State the forces and their directions on the lamp.

Write your reasoning before opening the solution

Hints

Hint 1: choose the decisive model
Name the selected body, then ask which external objects interact with it.
View solution step by step

  1. Compare your model with the physical interactions

    Method

    Include weight down, cable tension along the cable, and wind force right.

    Reason

    Earth, the cable and the moving air are the external interacting bodies.

    Working

    Do not add motion, a resultant arrow, or the lamp's pull on the cable.

Misconception contrast

The crate pushes the floor down, so that force belongs on the crate's diagram.
The downward push acts on the floor. The crate's diagram instead contains the upward normal force exerted by the floor on the crate.

Changed-context transfer

Challenge 3

Transfer the force model

Minimal support

Problem

A ball is rising after it has left a hand, and air resistance is not negligible. Construct its force model.

Commit to a model before using the hint

Hints

Hint 1: identify what changed
Once contact with the hand ends, decide which interactions still act on the ball.
View solution step by step

  1. Apply the model in the new context

    Method

    Draw weight downward and air resistance downward.

    Reason

    Both forces act on the ball; the hand no longer touches it, and upward velocity is not a force.

    Working

    Selected body: ball. External forces: weight and drag, both downward at that instant.

Independent check

Repair this diagram for a box sliding down a rough slope: it shows weight down the slope, normal force vertically up, friction down the slope and a separate resultant arrow.

Check the force-model reasoning

Weight acts vertically downward; normal force is perpendicular to the slope; friction acts up the slope because the surfaces slide relative to each other down the slope; the resultant is calculated, not drawn as an extra interaction force.

Start the Dynamics topic check. Use its feedback to practise the smallest weak step.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Combined Science Physics component
Edition
SEC G3 Combined Science Physics component 2027