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A Level · Cambridge (CIE) · 9709

Accelerate a particle on a rough horizontal surface

Resolve an angled pull, calculate its effect on the normal reaction and use the stated sliding-friction model in the horizontal Newton equation.

Mechanics · Newton’s laws and connected particles · 4.4
Exam scope and assessment notes

Mechanics (Paper 4, AS & A Level) · Mathematics 9709, 2026–2027, section 4.4. Use g = 10 m s⁻² throughout these problems. Each particle has positive constant mass. Forces and acceleration are constant within each stated stage; linear motion only. Use the resultant force in F = ma. A light taut inextensible string over a smooth pulley has one tension and equal acceleration magnitudes; those constraints cease if the string becomes slack. A rope can pull but cannot push; a light rigid tow-bar may exert tension or thrust. Resistance other than the stated friction is neglected unless explicitly supplied. In our rough-contact model, sliding friction has magnitude μR and opposes motion; at rest, friction adjusts with magnitude at most μR. Check contact and motion assumptions before continuing a solution. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Resolve forces using sine and cosine.
  • Use the smooth and rough contact models and check motion direction.

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

Quick readiness check

Which normal reaction applies to an upward angled pull on a horizontal particle in contact?

What you will learn

  • Use the perpendicular balance to find reaction before friction.
  • Calculate horizontal acceleration with a parallel or angled drive.
The 5 kg particle slides right under an upward-right pull of 25 N, with sine 3/5 and cosine 4/5. The pull components are 15 N up and 20 N right. Reaction is 35 N, and the coefficient 1/5 gives sliding friction 7 N left. The rightward resultant 13 N gives acceleration 2.6 m s⁻². Actual force arrows are schematic; components are used in the numerical equations rather than added as extra forces.An angled pull reduces reactionSliding right →; μ = 1/55 kgP = 25 NR = 35 NW = 50 N7 NR + 15 − 50 = 020 − 7 = 5a → a = +2.6 m s⁻²Schematic arrows; use numerical balances
The 5 kg particle slides right under an upward-right pull of 25 N, with sine 3/5 and cosine 4/5. The pull components are 15 N up and 20 N right. Reaction is 35 N, and the coefficient 1/5 gives sliding friction 7 N left. The rightward resultant 13 N gives acceleration 2.6 m s⁻². Actual force arrows are schematic; components are used in the numerical equations rather than added as extra forces.

The reaction can change when the drive is angled

For a particle constrained to a horizontal surface, vertical acceleration is zero while contact persists. A pull P at angle θ above the surface gives R + P sin θ − mg = 0. Thus R = mg − P sin θ; using R = mg would miss the lifting component. A downward angled push instead increases R. In either case check that the resulting normal force is non-negative before using the surface-contact model.

In our stated sliding model, friction has magnitude μR and opposes the current motion. For rightward motion under an upward-right pull, the horizontal equation is P cos θ − μR = ma. A 5 kg particle with P = 25 N, sin θ = 3/5, cos θ = 4/5 and μ = 1/5 has R = 50 − 15 = 35 N, friction 7 N and a = (20 − 7)/5 = 2.6 m s⁻² rightwards.

Use the force law appropriate to the current state

A simple horizontal drive with no vertical component leaves R = mg. For a moving 4 kg particle with forward force 18 N and μ = 0.2, friction is 8 N and a = (18 − 8)/4 = 2.5. A light rod can provide either a pulling or a pushing horizontal force; its direction must appear in the diagram rather than being decided from its name.

For a particle initially at rest, friction adjusts up to its limiting value instead of automatically equalling μR. A drive of 5 N on that 4 kg particle can be balanced by static friction 5 N, below the 8 N limit; the particle need not start moving. A negative acceleration during rightward sliding is valid, but after it stops, recheck static balance before extending the same sliding equation into leftward motion.

Pause and explain

A particle slides right while slowing. Which way does its sliding friction act?

Put the idea to work

Worked example

A 5 kg particle slides right under a 25 N pull with sin θ = 3/5 and cos θ = 4/5. The sliding coefficient is 1/5. Find reaction, friction and acceleration while contact and rightward motion persist.

Show the worked solution
  1. Vertical balance gives R = 50 − 25(3/5) = 35 N.
  2. Friction is (1/5) × 35 = 7 N leftwards; the pull’s horizontal component is 20 N.
  3. Therefore 20 − 7 = 5a, so a = 2.6 m s⁻² rightwards.

Answer R = 35 N; friction 7 N leftwards; a = +2.6 m s⁻².

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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 4 kg particle slides right on a rough horizontal surface under a horizontal 18 N drive. With μ = 0.2, find friction and acceleration.

Give me a hint

There is no vertical component in the drive, so first use R = mg.

Compare my reasoning
  1. Normal reaction is 40 N.
  2. Friction is 0.2 × 40 = 8 N leftwards.
  3. Horizontal balance gives 18 − 8 = 4a, so a = 2.5 m s⁻² rightwards.

Friction 8 N; a = +2.5 m s⁻².

Look for these in your work

  • Use the perpendicular balance before the friction law.
  • Subtract friction from the driving force.
2 · Independent

A 5 kg particle slides right with an upward-right pull 25 N, sin θ = 3/5, cos θ = 4/5 and μ = 1/5. Calculate R and acceleration.

Give me a hint

Use the vertical component to reduce R, then resolve along the surface.

Compare my reasoning
  1. R = 50 − 25(3/5) = 35 N.
  2. Friction is 7 N leftwards and horizontal pull is 25(4/5) = 20 N.
  3. Hence 5a = 13, so a = 2.6 m s⁻² rightwards.

R = 35 N; a = +2.6 m s⁻².

Look for these in your work

  • Do not assume reaction equals weight under this angled pull.
  • Use μR with the calculated reaction.
3 · Transfer

A 4 kg particle is initially at rest on a horizontal surface with static-friction limit μR, μ = 0.2. A horizontal 5 N pull is applied. Decide whether it must move and give the friction and acceleration in a possible resting state.

Give me a hint

Compare the friction needed for rest with its maximum available value.

Compare my reasoning
  1. R = 40 N, so the friction limit is 8 N.
  2. Rest requires friction 5 N opposing the pull, which is below 8 N and is admissible.
  3. The resting state has resultant zero and a = 0; motion is not forced by these data.

Rest is possible: friction 5 N opposing the pull; a = 0.

Look for these in your work

  • Use the static inequality rather than setting friction to its maximum.
  • Test the claimed motion before choosing a sliding branch.

Common mistakes

  • An angled force changes reaction and therefore friction.
  • Friction opposes motion, not necessarily acceleration.
  • Use a static-friction inequality for a resting particle.
Recall without your notes

What can you explain now?

Must a resting particle have friction equal to μR?

Compare with the explanation

No; static friction supplies the needed balance up to μR.

Equality describes the limiting state, not every resting contact.

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

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Make it stick

Resolve perpendicular first, choose static or sliding friction, then use the along-surface resultant in ma.

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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