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A Level · Cambridge (CIE) · 9709

Connect a rough horizontal particle to a hanging mass

Combine a surface friction calculation with the hanging particle’s gravity to find acceleration and tension, and test whether release starts motion.

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

  • Calculate static and sliding friction on a horizontal surface.
  • Solve ideal pulley equations with a common tension.

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

Quick readiness check

For a table particle whose string pulls horizontally, which normal reaction applies?

What you will learn

  • Resolve the surface particle before joining its equation to the hanging mass.
  • Distinguish a released resting system from a specified sliding state.
A 4 kg particle on a rough horizontal table is connected by a light taut inextensible string over a smooth pulley to a 2 kg hanging particle. Table weight and reaction are both 40 N; friction is 10 N away from the pulley. Tension is 50/3 N on each particle. Acceleration is 5/3 m s⁻² towards the pulley on the table and down on the hanging side. Arrows are schematic and the balances stop applying when the constraint changes.One system, two particle balances4 kg2 kgR = 40 NW = 40 NT = 50/3 N10 NT =50/3 N20 Na = 5/3 m s⁻²: table right, hanging downSchematic arrows; use numerical balances
A 4 kg particle on a rough horizontal table is connected by a light taut inextensible string over a smooth pulley to a 2 kg hanging particle. Table weight and reaction are both 40 N; friction is 10 N away from the pulley. Tension is 50/3 N on each particle. Acceleration is 5/3 m s⁻² towards the pulley on the table and down on the hanging side. Arrows are schematic and the balances stop applying when the constraint changes.

Friction on one particle resists the whole connected motion

A 4 kg particle on a rough horizontal table is connected over a smooth pulley to a hanging 2 kg mass by a light taut inextensible string. With μ = 1/4, the table reaction is 40 N and sliding friction is 10 N. If the 2 kg mass descends and the table particle slides towards the pulley, write T − 10 = 4a and 20 − T = 2a. Adding gives 10 = 6a, so a = 5/3 m s⁻² and T = 50/3 N.

The table particle’s weight and reaction balance vertically; they are not a horizontal driving force. The hanging weight supplies the gravitational drive along the string’s path. The common acceleration magnitude follows the taut inextensible connection; each particle’s diagram still needs its own external forces. After solving, 50/3 − 10 = 4(5/3) and 20 − 50/3 = 2(5/3) independently check the result.

At release, test static friction first

If the system starts at rest and the hanging weight is no more than the table’s static-friction limit, it may stay at rest. With 4 kg on the table, μ = 0.6 and 2 kg hanging, the maximum friction is 24 N. Rest needs T = 20 N and table friction 20 N, both consistent with the model. Setting friction to 24 N and predicting a reverse acceleration would choose a sliding branch that has not begun.

When hanging weight exceeds the static limit under our equal-coefficient model, the indicated descent starts and the sliding equations apply. If the table particle is already moving towards the pulley but gravity drive is weaker than sliding friction, acceleration can oppose the motion until it stops. Recheck the static state at that event. Equations must also change if the hanging particle hits the floor or the string slackens.

Pause and explain

A 4 kg table particle has μ = 0.6 and a 2 kg hanging partner. Released from rest, what is admissible?

Put the idea to work

Worked example

A 4 kg table particle is connected to a 2 kg hanging mass by an ideal pulley string. It moves towards the pulley as the hanging mass descends. With sliding μ = 1/4, find a and T before contact.

Show the worked solution
  1. Reaction is 40 N and sliding friction is 10 N.
  2. Equations T − 10 = 4a and 20 − T = 2a give a = 10/6 = 5/3 m s⁻².
  3. Substitution gives T = 10 + 4(5/3) = 50/3 N.

Answer a = 5/3 m s⁻² towards the pulley and down; T = 50/3 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 3 kg particle slides towards a pulley on a smooth horizontal table, connected to a descending 2 kg hanging mass by an ideal string. Find a and T.

Give me a hint

Smooth removes table friction, but both masses contribute to inertia.

Compare my reasoning
  1. For the table particle T = 3a; for the hanging mass 20 − T = 2a.
  2. Adding gives 20 = 5a, so a = 4 m s⁻².
  3. Then T = 3 × 4 = 12 N, satisfying both equations.

a = 4 m s⁻²; T = 12 N.

Look for these in your work

  • Use both particles in the summed inertia.
  • Retain tension in each individual balance.
2 · Independent

A 4 kg particle slides towards a smooth pulley on a rough horizontal table with μ = 1/4, connected to a descending 2 kg mass by a light taut string. Find acceleration and tension.

Give me a hint

Calculate the table reaction and friction before adding the along-motion balances.

Compare my reasoning
  1. Reaction is 40 N and friction is 10 N.
  2. T − 10 = 4a and 20 − T = 2a give a = 5/3 m s⁻².
  3. T = 10 + 4(5/3) = 50/3 N.

a = 5/3 m s⁻²; T = 50/3 N.

Look for these in your work

  • Use friction opposite the stated table motion.
  • Check both the table and hanging equations after substitution.
3 · Transfer

A 4 kg table particle is connected to a 2 kg hanging mass, initially at rest. The static-friction limit is μR with μ = 0.6. Decide whether rest is possible and find its tension and friction.

Give me a hint

Test the friction required by equilibrium against the available maximum.

Compare my reasoning
  1. The table reaction is 40 N and static-friction limit is 24 N.
  2. The hanging mass can rest with T = 20 N.
  3. The table then requires friction 20 N opposing tension, which is below 24 N; a = 0 is admissible.

Rest is possible: T = 20 N; friction 20 N; a = 0.

Look for these in your work

  • Avoid using the sliding maximum for an unstarted system.
  • Check the equilibrium of both particles.

Common mistakes

  • The table weight is not an along-table drive.
  • Use a static test for a system released from rest.
  • Check the equations only while the connection is taut and before contact.
Recall without your notes

What can you explain now?

If the table’s static limit exceeds the hanging weight, must a system released from rest move?

Compare with the explanation

No; suitable static friction can hold the system at rest.

Tension equals the hanging weight in that rest state, and the table needs only that friction value.

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

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Make it stick

Find contact friction on the table, combine both balances and check whether the assumed motion can start or persist.

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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