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A Level · Cambridge (CIE) · 9709

Distinguish static friction from its limiting value

Find the friction actually required for equilibrium, compare it with μR and use equality only when the particle is about to slip.

Section 4.1 · Forces and equilibrium

Mechanics · Forces and equilibrium · 4.1
Exam scope and assessment notes

Mechanics (Paper 4, AS & A Level) · Mathematics 9709, 2026–2027, section 4.1. Paper 1 algebra and trigonometry are assumed. Bodies are modelled as particles: forces act at one point and rotational effects are not considered. Resolve numerically; a scale drawing is not a substitute for calculation. Force diagrams are qualitative unless stated otherwise. Where mass is supplied in this sequence, use the stated g = 10 m s⁻². Check non-negative tension and contact reaction and use F ≤ μR except at limiting friction. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Calculate normal reaction and required tangential balance.
  • Solve linear inequalities with non-negative force magnitudes.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

What does ‘about to slip’ mean in this model?

What you will learn

  • Use the coefficient of friction as limiting friction divided by normal reaction.
  • Choose F ≤ μR or F = μR according to the stated rest or limiting condition.

At rest friction adjusts to the required balance

In the supplied dry-friction model, the magnitude of static friction satisfies 0 ≤ F ≤ μR, where μ is the dimensionless coefficient of friction and R is normal reaction. At limiting friction, F = μR; this is the state described as ‘about to slip’ or limiting equilibrium. A particle merely stated to be at rest need not be at that limit. Find the friction needed from tangential balance first, then test it against the capacity.

For weight 50 N on a horizontal rough plane with μ = 0.4 and a 12 N horizontal pull, R = 50 N and the capacity is 20 N. Equilibrium requires F = 12 N opposite the pull, not 20 N. The spare capacity is 8 N. At zero horizontal pull the required friction is zero, despite the same non-zero capacity. Friction is not a constant extra force that always acts at maximum strength.

Use equality only at the limiting state

If a particle at limiting friction has R = 60 N and F = 18 N, then μ = 18/60 = 0.3. The coefficient has no units because it is a ratio of force magnitudes. For that same reaction, a later equilibrium requiring only 12 N friction is below the limit. A measured F/R at a non-limiting rest state does not in general equal μ; it provides only a lower bound on the coefficient needed to support it.

For weight 100 N with μ = 0.35 and a non-negative horizontal pull P, maintained horizontal-surface equilibrium requires P = F ≤ 35 N. Thus 0 ≤ P ≤ 35 N; P = 35 is the limiting case. If P exceeds this, the assumed rest state is impossible under the friction model. Describe the failure of equilibrium here; acceleration and moving-particle laws belong to the later Dynamics section.

Pause and explain

A resting particle needs 12 N friction while μR = 20 N. What is actual F?

Put the idea to work

Worked example

A particle of weight 50 N rests on a horizontal rough plane with μ = 0.4 under a horizontal pull of 12 N. Find actual friction, its limiting value and spare capacity.

Show the worked solution
  1. Vertical balance gives R = 50 N.
  2. Horizontal balance requires friction 12 N opposite the pull.
  3. The limiting value is μR = 20 N, so the rest state is possible and spare capacity is 8 N.

Answer Actual F = 12 N; limiting friction 20 N; spare capacity 8 N.

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From guided practice to a new situation

Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.

1 · Guided

A particle of weight 80 N rests on a horizontal rough plane, μ = 0.25, under a 14 N horizontal pull. Find R, actual F, maximum F and spare capacity.

Give me a hint

Find the required balance before applying the capacity inequality.

Compare my reasoning
  1. The reaction is 80 N and actual friction is 14 N against the pull.
  2. Maximum friction is 0.25 × 80 = 20 N.
  3. Since 14 ≤ 20 the rest state is supported, with spare capacity 6 N.

R = 80 N; actual F = 14 N; maximum F = 20 N; spare capacity 6 N.

Look for these in your work

  • Distinguish required friction and limiting friction.
  • Check the inequality and remaining capacity.
2 · Independent

A contact is at limiting friction with R = 60 N and F = 18 N. Find μ. For the same R and coefficient, what actual friction acts when a rest state requires 12 N tangential resistance?

Give me a hint

Use the limiting observation to determine μ, then use balance for the later state.

Compare my reasoning
  1. At the limit, μ = 18/60 = 0.3.
  2. The capacity at R = 60 remains 18 N.
  3. The later required 12 N is below capacity, so actual friction is 12 N rather than 18 N.

μ = 0.3; later actual friction 12 N, below the 18 N limit.

Look for these in your work

  • Use equality only in the supplied limiting observation.
  • Let actual static friction adjust in the later equilibrium.
3 · Transfer

A 100 N particle rests on a horizontal rough plane with μ = 0.35. A horizontal rightward pull P increases from zero. Find the full non-negative range permitting equilibrium and describe its endpoints.

Give me a hint

Reaction is unchanged by a purely horizontal pull.

Compare my reasoning
  1. R = 100 N, so friction capacity is 35 N.
  2. Equilibrium needs leftward F = P, giving 0 ≤ P ≤ 35 N.
  3. At P = 0 friction is zero; at P = 35 N it is limiting; above 35 the rest model fails.

0 ≤ P ≤ 35 N; zero friction at 0, limiting friction at 35 N.

Look for these in your work

  • Use a force inequality for the whole range.
  • Explain both endpoints without assuming all rest states are limiting.

Common mistakes

  • Do not set F = μR merely because the particle is at rest.
  • Use the normal reaction, which need not equal weight.
  • A coefficient is dimensionless; a below-limit F/R gives a bound rather than its exact value.
Recall without your notes

What can you explain now?

At rest with no tangential applied force on a horizontal plane, must friction be μR?

Compare with the explanation

No: required friction is zero.

The surface’s capacity does not force friction to act when no tangential balance requires it.

After trying it yourself, choose your next review. This is your self-assessment.

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Make it stick

Find required friction, compare with μR and reserve equality for a stated or calculated limiting condition.

If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.

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Course scope and assessment

312 sequenced lessons provide 936 guided, independent and application problems covering the 160 objectives in sections 1.1–1.8, 2.1–2.6, 3.1–3.9, 4.1–4.5, 5.1–5.5 and 6.1–6.5. The map lists all 38 sections across six alternative components; all eight Pure Mathematics 1 sections and all six Pure Mathematics 2 sections, from Paper 2 Algebra through Numerical solution of equations, currently have full teaching sequences. Paper 3 Algebra, section 3.1, links the six shared foundation lessons plus six lessons on partial fractions and rational binomial expansions; Paper 3 logarithms, trigonometry and numerical equations use the shared lessons, with the Paper 1 graph foundation linked into trigonometry. Paper 3 differentiation combines the shared methods with its inverse-tangent derivative lesson. Paper 3 Integration links four shared standard and identity lessons plus seven advanced lessons, covering all six section 3.5 requirements. Eight Vectors lessons cover all six section 3.7 requirements, with 2D and 3D line and solid applications. Nine Differential equations lessons cover all four section 3.8 requirements, including Paper 3 integration techniques, initial conditions, equilibrium solutions and conditional model interpretations. Eleven Complex numbers lessons cover all eight section 3.9 requirements, with seven Argand diagrams and explicit argument and boundary checks. All nine Pure Mathematics 3 sections now have teaching linked to every mapped requirement. Nine Forces and equilibrium lessons cover all seven section 4.1 requirements, with six force diagrams, signed incline-friction intervals, reaction-dependent pull and push limits and separate-body third-law checks. Nine Kinematics lessons cover the four official section 4.2 requirements, with the graph requirement mapped to three relationships. Six motion diagrams support directed quantities, graph slopes and signed areas, Paper 1 time-calculus methods, reversal distances, constant-acceleration stages and shared-clock particle meetings. Six Momentum lessons cover both section 4.3 requirements, including signed system totals, conservation assumptions, outgoing velocities, coalescence, unknown masses and successive impacts. Four diagrams show system momentum through the stipulated events, and supplied kinetic-energy checks distinguish conservation of momentum from changes in kinetic energy. Twelve Newton’s laws lessons cover all four section 4.4 requirements, with vertical support and release stages, rough horizontal and incline motion, inferred parameters, hanging and surface pulley systems, rigid tow-bar thrust and slack-string transitions. Seven force diagrams support the separate-particle balances. Nine Energy, work and power lessons cover all five section 4.5 requirements, with angle-dependent work, kinetic and reference potential energy, signed net-work balances, smooth curved slides, rough and driven slopes, ideal connected-system energy and instantaneous power on hills. Five diagrams distinguish the energy inputs, losses and valid positive-speed force model. All five Mechanics sections now have teaching and staged practice linked to their mapped requirements. Thirteen Representation of data lessons cover all five section 5.1 requirements, including choice and limitations of displays, ordered and back-to-back stems, raw summaries and box plots, unequal-width density histograms, cumulative-frequency interpolation, descriptive moment calculations, grouped estimates, coding, combining two datasets and correcting totals. Six accessible diagrams retain repeated observations and distinguish numerical spread, frequency area and estimated readings. Ten Permutations and combinations lessons cover both official section 5.2 requirements, with selection and ordered roles, repeated objects, adjacency blocks, non-adjacency gaps, leading-zero cases, labelled multi-row seating and mixed restrictions. Six accessible diagrams show outcome trees, ordered versus unordered choices, repeated copies, blocks, gaps and distinct row seats. Eleven Probability lessons cover all four section 5.3 requirements, with equiprobable elementary outcomes, selection-based probabilities, complements and disjoint regions, independence tests, restricted sample spaces, two-way tables, replacement and removal trees, reversed conditions and unknown parameters. Seven accessible diagrams show dice outcomes, random subsets, event regions, a conditioned grid, a count table and two probability trees. Twelve Discrete random variables lessons cover all three official section 5.4 requirements, with complete probability tables, moment and parameter checks, binomial assumptions, exact probabilities and moments, geometric first-success probabilities, tails, expectation and integer reliability thresholds. Six accessible diagrams distinguish outcome grouping, weighted moments, fixed trials, discrete probability bars, waiting conventions and integer tail deadlines. Twelve Normal distribution lessons cover all three official section 5.5 requirements, with continuous-area interpretation, MF19 cumulative tables, full standardisation, tail and interval regions, conditional probabilities, percentiles, one- and two-parameter inference, binomial suitability, continuity corrections and approximate integer thresholds. Seven accessible diagrams show model areas, table reading, aligned scales, intervals, quantiles, matched binomial and normal shapes and whole integer cells. All five Statistics 1 sections now have teaching and staged practice linked to their mapped requirements. Fourteen Poisson distribution lessons cover all five official section 6.1 requirements, with homogeneous event-count assumptions, exposure scaling, exact masses and tails, conditional count events, moments, parameter inference, probability duration targets, disjoint and overlapping windows, rare-count binomial approximations, matched normal approximations and continuity corrections. Nine accessible diagrams show event exposures, unit conversions, discrete bars, complete tail complements, population moments, shared windows, approximate count models and integer cells. Exact checks distinguish approximate and exact close-target decisions. Fourteen Linear combinations lessons cover the six listed results in official section 6.2, with affine moments, signed scales and constant cases, expectation without independence, independent squared variance contributions, fixed and random fees, independent copies versus reused values, affine normal events, independent normal sums and differences, weighted and conditional areas, coefficient inference, exact independent Poisson sums and overlapping exposure limits. Nine accessible diagrams distinguish scale changes, variance contributions, component costs, copy mechanisms, aligned normal scales, summed areas, Poisson count pairs and unsupported distribution claims. Fourteen Continuous random variables lessons cover both official section 6.3 requirements, with density validity and units, support-aware intervals, complements and conditional regions, first and second moments, percentiles by direct density areas, symmetry checks, parameter inference, infinite-support power and exponential integrals, divergent-moment limits and combined applications. Nine accessible diagrams distinguish density height from probability, normalisation, negative-density failures, central and conditional areas, mean versus median, percentile locations and partial infinite tails. Explicit cumulative distribution function knowledge is not required. Eighteen Sampling and estimation lessons cover all eight official section 6.4 requirements, with population and frame definitions, random-number selection, sampling-quality limits, independent sample-mean moments, normal means, CLT conditions, SE scaling, raw, summarised and coded unbiased estimates, known-variance and large-sample mean intervals, repeated-sampling interpretation, precision planning, proportion intervals, sparse-outcome limits and contextual applications. Ten accessible diagrams distinguish selection frames, number rejection, repeated sample means, population versus mean density, CLT models, variance divisors, interval targets, illustrative coverage, integer size checks and proportion margins. Eighteen Hypothesis tests lessons cover all five official section 6.5 requirements, with pre-specified alternatives, inclusive count tails, explicit equal-tail conventions, integer critical regions and actual significance, exposure-aware Poisson tests, continuity-corrected count approximations, normal and large-sample mean tests, confidence-interval connections, error classifications, fixed-region binomial, Poisson and normal error probabilities and sample-size power comparisons. Nine accessible diagrams show the testing procedure, exact count regions, unequal Poisson tails, discrete significance jumps, corrected count boundaries, matching mean intervals and alternative error distributions. All shared lessons are counted once in the course total. All 38 sections now link teaching and practice. Eight earlier overview lessons remain available, but do not count as full section coverage. This is a content-availability measure, not full programme completion or exam readiness. Written practice is self-checked, not officially marked. Check the 2028–2030 syllabus separately if sitting an examination from 2028; this teaching map uses 2026–2027 requirements.

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