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Resultant force and balanced motion

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

The resultant force is the vector sum of all forces on one body. Zero resultant means zero acceleration: the body stays at rest or continues with constant velocity. A non-zero resultant changes speed, direction, or both.

Worked reasoning

Modelled example 1

Model the decisive reasoning

Core

Problem

A cyclist moves right at constant velocity. Driving force is 120 N right and resistance is 120 N left. Explain the motion.
Study the worked solution

  1. Select and apply the model

    Method

    Add the horizontal forces with direction to find a zero resultant.

    Reason

    Zero resultant force gives zero acceleration, not necessarily zero velocity.

    Working

    Fnet = 120 N - 120 N = 0 N; the cyclist continues at constant velocity.

Guided practice 2

Complete a guided force model

About 5 min

Problem

A trolley has 18 N right and 7 N left. Determine the resultant and what happens to its velocity.

Write your reasoning before opening the solution

Hints

Hint 1: choose the decisive model
Choose right as positive and combine the two forces before describing motion.
View solution step by step

  1. Compare your model with the physical interactions

    Method

    The resultant is 11 N right, so the trolley accelerates right.

    Reason

    A non-zero resultant changes velocity in the resultant's direction.

    Working

    Fnet = +18 N - 7 N = +11 N.

Misconception contrast

A moving body must have a forward resultant force.
A forward resultant is required for forward acceleration. A body can move at constant velocity while real forces balance to a zero resultant.

Changed-context transfer

Challenge 3

Transfer the force model

Minimal support

Problem

A satellite changes direction while its speed stays constant. Is the resultant force zero?

Commit to a model before using the hint

Hints

Hint 1: identify what changed
Velocity includes direction as well as speed.
View solution step by step

  1. Apply the model in the new context

    Method

    The resultant is non-zero and points in the direction of the acceleration.

    Reason

    Changing direction changes velocity even when speed is unchanged.

    Working

    Constant speed does not imply constant velocity or zero resultant force.

Independent check

A boat has 540 N thrust east, 410 N water resistance west and 130 N wind force west. Determine the resultant and describe its motion if it was already moving east.

Check the force-model reasoning

The westward forces total 540 N, so the resultant is zero. Acceleration is zero and the boat continues east at constant velocity.

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