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Friction and resistive effects

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

Friction and drag are interaction forces. Determine their direction from relative motion or its tendency, include them in the resultant, and then explain the effect on acceleration or energy transfer.

Worked reasoning

Modelled example 1

Model the decisive reasoning

Core

Problem

A box is pushed right but remains at rest on a rough floor. Explain the friction force.
Study the worked solution

  1. Select and apply the model

    Method

    Static friction acts left and matches the gentle push while the box remains at rest.

    Reason

    It opposes the tendency of the box to slide right relative to the floor.

    Working

    Zero motion can still involve non-zero friction and a zero resultant.

Guided practice 2

Complete a guided force model

About 5 min

Problem

A car has 900 N driving force and 300 N total resistance. State how resistance affects acceleration.

Write your reasoning before opening the solution

Hints

Hint 1: choose the decisive model
Include resistance before using Fnet = ma.
View solution step by step

  1. Compare your model with the physical interactions

    Method

    Resistance leaves a 600 N forward resultant.

    Reason

    Drag and rolling resistance oppose the car's forward motion relative to its surroundings.

    Working

    Fnet = 900 - 300 = 600 N forward; acceleration follows from this resultant.

Misconception contrast

Friction always acts opposite the velocity of the whole object.
Friction opposes relative sliding or its tendency at the contact. During walking, the ground's friction on a foot can act forward while the person moves forward.

Changed-context transfer

Challenge 3

Transfer the force model

Minimal support

Problem

A bus brakes on a road. Explain one useful effect and one unwanted effect of friction.

Commit to a model before using the hint

Hints

Hint 1: identify what changed
Separate the tyre-road grip from dissipative heating and wear.
View solution step by step

  1. Apply the model in the new context

    Method

    Tyre-road friction provides the backward force that slows the bus; friction also heats and wears the tyres and brakes.

    Reason

    The same interaction can control motion usefully while transferring energy to internal energy stores.

    Working

    Useful: grip and braking. Unwanted: heating and material wear.

Independent check

A foot pushes backward on the ground without slipping. State the direction of friction on the foot and explain why this does not contradict the friction rule.

Check the force-model reasoning

The foot tends to slide backward relative to the ground, so static friction exerted by the ground acts forward on the foot. It opposes relative slipping, not necessarily the person's forward 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