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

Use smooth contact on inclined planes and vertical walls

Set friction to zero, resolve weight in surface directions and test whether the remaining forces can support 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

  • Resolve vertical weight parallel and normal to an incline.
  • Use two perpendicular component equilibrium equations.

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

Quick readiness check

For an incline θ above the horizontal, what is the weight’s downslope component?

What you will learn

  • Solve smooth-contact equilibrium with forces along or oblique to a plane.
  • Explain what the smooth-particle model omits and when it cannot support rest.

Smooth does not mean no contact force

A smooth surface supplies normal reaction but no friction. On a plane inclined at θ to the horizontal, weight W has downslope component Wsin θ and into-plane component Wcos θ. If a taut string parallel to the plane supports the particle, equilibrium gives T = Wsin θ and R = Wcos θ. A 50 N weight on a smooth 30° plane therefore needs T = 25 N and R = 25√3 N.

A smooth particle on a non-horizontal plane without another upslope force cannot remain at rest, because Wsin θ is unbalanced. Setting R = W would not fix that missing parallel force. The smooth model neglects tangential resistance; a real surface may not satisfy it exactly. It is useful only while the stated contact and applied-force assumptions remain appropriate, and the particle model omits possible tipping of an extended object.

An oblique support force also changes the reaction

A horizontal force P pushing a particle towards the uphill side of a smooth plane has upslope component Pcos θ and into-plane component Psin θ. Rest requires Pcos θ = Wsin θ, so P = Wtan θ; the normal reaction is R = Wcos θ + Psin θ = W/cos θ. With W = 30 N and θ = 30°, P = 10√3 N and R = 20√3 N. The extra horizontal push increases the contact reaction.

At a smooth vertical wall, its normal reaction is horizontal and cannot support vertical weight. A 40 N particle pressed against the wall horizontally by 15 N has R = 15 N, but still needs a vertical upward force of 40 N, such as a taut vertical string. Without that support it cannot stay at rest in this model. Contact alone is not a guarantee of equilibrium in all directions.

Pause and explain

What can a smooth vertical wall’s reaction balance?

Put the idea to work

Worked example

A 50 N particle is held on a smooth 30° incline by a string parallel to the plane. Find T and R.

Show the worked solution
  1. Resolve weight: 50sin30° = 25 N downslope and 50cos30° = 25√3 N into the plane.
  2. Parallel balance gives T = 25 N upslope.
  3. Normal balance gives R = 25√3 N away from the plane; smooth contact supplies no friction.

Answer T = 25 N; R = 25√3 N ≈ 43.3 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

An 80 N particle is at rest on a smooth 30° incline with a string parallel to the plane. Find tension and reaction.

Give me a hint

Balance independently along and normal to the plane.

Compare my reasoning
  1. Downslope weight is 80sin30° = 40 N.
  2. Hence tension is 40 N upslope.
  3. Normal reaction is 80cos30° = 40√3 N; no friction is included.

T = 40 N; R = 40√3 N ≈ 69.3 N.

Look for these in your work

  • Use the correct weight components.
  • Apply the zero-friction assumption in the parallel equation.
2 · Independent

A 30 N particle rests on a smooth 30° incline under a horizontal force P towards the uphill side. Find P and R.

Give me a hint

The horizontal force has an upslope component and an into-plane component.

Compare my reasoning
  1. Parallel balance gives Pcos30° = 30sin30°, so P = 10√3 N.
  2. Normal balance gives R = 30cos30° + Psin30°.
  3. Substitution gives R = 15√3 + 5√3 = 20√3 N.

P = 10√3 N; R = 20√3 N.

Look for these in your work

  • Resolve the horizontal force in both plane directions.
  • Check that the push increases the normal reaction.
3 · Transfer

A 40 N particle rests against a smooth vertical wall under a horizontal force 15 N into the wall and a taut vertical string. Find R and T, and explain why removing the string prevents rest.

Give me a hint

Separate the wall-normal horizontal balance from the vertical support balance.

Compare my reasoning
  1. Horizontal balance gives R = 15 N away from the wall.
  2. Vertical balance gives T = 40 N upwards.
  3. Without the string, weight is unbalanced because smooth contact provides no friction and its normal force is horizontal.

R = 15 N; T = 40 N; without the string the smooth-wall model cannot support rest.

Look for these in your work

  • Use the correct direction of the wall reaction.
  • Explain the missing upward force under the changed model.

Common mistakes

  • Smooth contact removes friction but not normal reaction.
  • Weight’s plane components are Wsin θ along and Wcos θ normal to the incline.
  • A wall’s horizontal normal cannot balance vertical weight.
Recall without your notes

What can you explain now?

Can a smooth inclined plane alone hold a particle at rest when θ > 0 and W > 0?

Compare with the explanation

No: the downslope component Wsin θ is unbalanced.

The normal reaction has no component along the plane, and friction is absent.

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

Set friction to zero, resolve every force along and normal to a surface and test whether both directions can balance under a smooth-contact model.

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.