Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9709

Separate contact into normal reaction and friction

Resolve a surface contact force into perpendicular reaction and tangential friction, and calculate how an angled pull or push changes the reaction.

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

  • Resolve oblique forces and use equilibrium.
  • Calculate a resultant from perpendicular components.

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

Quick readiness check

How is normal reaction directed relative to a surface?

What you will learn

  • Identify the two components of a rough surface’s contact force.
  • Calculate reaction and contact resultant using the supplied applied-force directions.

A rough surface’s total force has two components

Represent contact by a normal reaction R perpendicular to the surface and friction F along it. For maintained non-adhesive contact, R ≥ 0. The combined contact-force magnitude is √(R² + F²), not generally R alone. If F is non-zero, the resultant is inclined to the normal by an angle with tangent |F|/R, when R > 0. The friction direction depends on the tendency for relative sliding.

A 5 kg particle on a horizontal surface is pulled right by 20 N at 30° above the horizontal and held at rest by contact. With g = 10 m s⁻², vertical balance gives R + 20sin30° = 50, so R = 40 N. Horizontal balance requires friction 20cos30° = 10√3 N left. The total contact force has magnitude √(40² + (10√3)²) = 10√19 N. Available friction still has to be sufficient for the proposed equilibrium.

Pulling up and pushing down have opposite normal effects

An upward component of an applied force reduces the normal reaction, whereas a downward component increases it. A 4 kg particle pushed right at 10 N and 30° below the horizontal has R = 40 + 10sin30° = 45 N and requires friction 5√3 N left for rest. Using R = mg in either angled case would miss the vertical contribution and produce the wrong friction limit later.

On an incline, R is perpendicular to the plane, not vertical. Resolve all forces normal to that plane before finding it. If an upward pull calculation gives R < 0, a non-adhesive surface cannot exert the needed downward contact: maintained contact is impossible under that arrangement. At R = 0 the body is at the contact boundary; the dry-friction model gives no friction capacity there. A force balance that needs both negative R and positive friction is not physically valid.

Pause and explain

A horizontal-surface pull has upward component U and weight W. What is R for vertical balance?

Put the idea to work

Worked example

A 5 kg particle is at rest on a rough horizontal surface under a 20 N pull at 30° above the horizontal. Use g = 10 m s⁻². Find R, required F and the contact-force magnitude.

Show the worked solution
  1. Vertical balance gives R = 50 − 20sin30° = 40 N.
  2. Horizontal balance requires F = 20cos30° = 10√3 N left.
  3. The perpendicular contact components give magnitude √(1600 + 300) = 10√19 N.

Answer R = 40 N; required F = 10√3 N left; contact magnitude 10√19 N ≈ 43.6 N.

Try a topic test

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

Test this topic →

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 4 kg particle is at rest on a rough horizontal surface under a 10 N push at 30° below the rightward horizontal. Use g = 10 m s⁻². Find R and required friction.

Give me a hint

The push adds a downward component to the weight.

Compare my reasoning
  1. Weight is 40 N and downward push component is 10sin30° = 5 N.
  2. Vertical balance gives R = 45 N.
  3. The rightward push component is 10cos30° = 5√3 N, requiring that friction leftwards.

R = 45 N; required friction 5√3 N left.

Look for these in your work

  • Use the downward sign of the push component.
  • Find the tangential force separately from reaction.
2 · Independent

A rough contact has normal component 24 N and tangential friction component 7 N. Find the total contact magnitude and its angle to the normal.

Give me a hint

The components are perpendicular, so use Pythagoras and the correct angle ratio.

Compare my reasoning
  1. Contact magnitude is √(24² + 7²) = 25 N.
  2. If β is measured from the normal towards friction, tan β = 7/24.
  3. Hence β ≈ 16.3°, on the side indicated by the friction direction.

Contact magnitude 25 N; angle 16.3° from the normal towards friction.

Look for these in your work

  • Use the normal and tangential components as perpendicular legs.
  • Specify the reference direction for the angle.
3 · Transfer

A 20 kg particle is at rest on a rough horizontal surface under a 100 N pull at 30° above the rightward horizontal. Use g = 10 m s⁻². Find R and required F and explain why friction capacity cannot be certified without μ.

Give me a hint

Balance the upward pull component before calculating the tangential force.

Compare my reasoning
  1. Weight is 200 N, so R = 200 − 100sin30° = 150 N.
  2. Horizontal balance needs F = 100cos30° = 50√3 N left.
  3. Rest requires 50√3 ≤ 150μ; without μ the available friction capacity is not established.

R = 150 N; required F = 50√3 N left; equilibrium requires μ ≥ 1/√3.

Look for these in your work

  • Include the normal effect of the pull.
  • Distinguish a required friction force from proof that the surface can supply it.

Common mistakes

  • R is a contact component, not the full rough-contact resultant.
  • Include upward and downward applied components when calculating R.
  • Reject negative reaction for a non-adhesive contact.
Recall without your notes

What can you explain now?

Does an upward angled pull leave R = mg on a horizontal surface?

Compare with the explanation

Not generally; its upward component reduces R.

Vertical balance includes every force component, not only weight and contact.

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

Your review choice appears on Today. Sign in to sync it across devices.

Make it stick

Separate rough contact into R and F, resolve the other forces and check both contact signs and any required friction capacity.

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.

Keep learning Mathematics

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.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.