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A Level · Cambridge (CIE) · 9709

Join vertical force stages with constant acceleration motion

Turn cable forces into acceleration, calculate a stopping point and change the model when a cable is released.

Section 4.4 · Newton’s laws of 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

  • Use weight and tension in a vertical balance.
  • Apply v = u + at and v² = u² + 2as with signed quantities.

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

Quick readiness check

A 4 kg particle is pulled up by 48 N. With g = 10, what is the upward resultant?

What you will learn

  • Combine a vertical Newton equation with constant-acceleration formulae.
  • Carry the reached velocity into a released free-motion stage.

Find acceleration before choosing a motion formula

A 4 kg particle lifted by a constant upward cable force 48 N has weight 40 N. With up positive, 48 − 40 = 4a, so a = +2 m s⁻² while that force acts. Starting from rest for 3 s gives v = 6 m s⁻¹ and upward displacement 9 m. The cable force is 48 N, not the resultant 8 N; substituting 48/4 directly as acceleration would omit weight.

If a 5 kg particle moving upward at 6 m s⁻¹ has cable tension 40 N, the resultant is downward: a = (40 − 50)/5 = −2. It comes momentarily to rest after 3 s and rises 9 m. Momentary rest is a kinematic event, not proof of force equilibrium; if the same positive tension continues, downward motion follows. A cable that would require negative tension needs a different, slack-string model.

Release changes forces without resetting velocity

Releasing the 4 kg particle after its 3 s lifting stage removes cable tension. With no resistance, the next stage has a = −10 m s⁻² but starts with u = +6 m s⁻¹. It rises a further 6²/(2 × 10) = 1.8 m before stopping at the highest point, reached 0.6 s after release. The release itself is stipulated to produce no instantaneous velocity change.

Use a local time origin and the reached height for each new stage. While the particle is free, acceleration remains downwards at the highest point where velocity is zero. Continue only over the interval requested and before any stated contact or impact. If a model includes air resistance, use its specified force rather than silently inserting a new resistance or assuming the free-fall acceleration.

Pause and explain

Immediately after release at upward velocity 6 m s⁻¹, which next-stage data apply without resistance?

Put the idea to work

Worked example

A 4 kg particle starts from rest under an upward 48 N cable force for 3 s, then is released without an instantaneous velocity change. Neglect resistance. Find its maximum rise from the start.

Show the worked solution
  1. During lifting, a = (48 − 40)/4 = 2 m s⁻²; after 3 s, v = 6 and rise is 9 m.
  2. After release, a = −10 and u = 6, so the extra rise is 6²/20 = 1.8 m.
  3. Total maximum rise is 9 + 1.8 = 10.8 m; it occurs 3 + 0.6 = 3.6 s after starting.

Answer Maximum rise 10.8 m at total time 3.6 s.

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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 starts from rest, pulled upward by a constant 36 N for 2 s. Find acceleration, velocity and rise.

Give me a hint

Use the net vertical force before the motion formulae.

Compare my reasoning
  1. Weight is 30 N, so a = (36 − 30)/3 = 2 m s⁻².
  2. Velocity is v = 0 + 2 × 2 = 4 m s⁻¹ upwards.
  3. Rise is s = ½ × 2 × 2² = 4 m.

a = +2 m s⁻²; v = +4 m s⁻¹; rise 4 m.

Look for these in your work

  • Include weight in the force equation.
  • Use the stated 2 s duration with the derived constant acceleration.
2 · Independent

A 5 kg particle moves upward at 6 m s⁻¹ with constant upward cable tension 40 N. Find the time and extra rise to its first instant of rest.

Give me a hint

The upward-positive acceleration is negative because tension is below weight.

Compare my reasoning
  1. Acceleration is a = (40 − 50)/5 = −2 m s⁻².
  2. Set v = 0: 0 = 6 − 2t, so t = 3 s.
  3. Using v² = u² + 2as gives 0 = 36 − 4s, so rise is 9 m.

a = −2 m s⁻²; stopping time 3 s; rise 9 m.

Look for these in your work

  • Keep tension positive while the resultant is negative.
  • Stop at the requested first zero velocity, not at assumed equilibrium.
3 · Transfer

A 2 kg particle starts at rest under upward cable force 30 N for 2 s. The cable is then released without a velocity jump. Neglect resistance. Find the greatest height above its starting level and total time to that point.

Give me a hint

Use separate accelerations and carry the speed reached after the first 2 s.

Compare my reasoning
  1. During the pull, a = (30 − 20)/2 = 5, so after 2 s velocity is 10 m s⁻¹ and rise is 10 m.
  2. After release, a = −10; the extra time is 10/10 = 1 s and extra rise is 10²/20 = 5 m.
  3. Total maximum height is 15 m, reached after 3 s; gravitational acceleration is still downward there.

Greatest rise 15 m; total time 3 s.

Look for these in your work

  • Change acceleration when tension disappears.
  • Add displacements and elapsed times from both correctly joined stages.

Common mistakes

  • Subtract weight from an upward cable force.
  • Carry velocity into the release stage.
  • At maximum height, velocity is zero but free acceleration remains downward.
Recall without your notes

What can you explain now?

Does a released particle acquire zero velocity immediately when its cable force vanishes?

Compare with the explanation

No; the reached velocity continues at release under the stated model.

The changed resultant determines the subsequent acceleration, not an instantaneous reset of velocity.

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Make it stick

Force balance gives each stage’s acceleration; carry its final velocity into the next force 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.

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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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