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A Level · Cambridge (CIE) · 9709

Infer a missing force mass or friction coefficient

Rearrange a Newton equation using measured acceleration, then reject parameters that contradict the stated motion and contact model.

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

  • Write Newton equations for horizontal and inclined sliding.
  • Rearrange linear equations and interpret negative results.

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

Quick readiness check

A 5 kg particle accelerates at 4 m s⁻² under drive 30 N and opposing resistance Q. Which equation is correct?

What you will learn

  • Recover an unknown force, mass or coefficient from a specified motion.
  • Check positive mass, non-negative friction and the assumed force directions.

Measured acceleration supplies the required resultant

A 5 kg particle driven horizontally by 30 N with forward acceleration 4 m s⁻² needs resultant 20 N. If the only opposition is a constant resistance Q, then 30 − Q = 20, so Q = 10 N. Do not treat Q = ma: the resistance is only one part of the resultant. For an unknown mass under drive 42 N and resistance 6 N with acceleration 3, 42 − 6 = 3m gives m = 12 kg.

For downhill sliding on a rough slope, mg sin θ − μmg cos θ = ma. A 4 kg particle with sin θ = 3/5, cos θ = 4/5 and downward a = 2 gives 24 − 32μ = 8, so μ = 1/2. Substituting it back recovers friction 16 N and resultant 8 N downwards. The inferred coefficient applies to the specified friction model, not automatically to all real motion conditions.

An algebraic result must satisfy the model

If that same gravity-only downhill particle is reported to accelerate at 8 m s⁻², the equation gives 24 − 32μ = 32 and μ = −1/4. A negative coefficient is incompatible with a resistive friction model. The reported data require a different force model, corrected measurements or corrected information; taking the absolute value would silently change the equation and would not reproduce the acceleration.

Negative signed acceleration itself is often valid; it can mean a particle moving in the positive direction is slowing. Negative mass or a negative friction magnitude is a different kind of issue. Distinguish a signed component from a non-negative physical parameter and state the interval for which the assumed direction of sliding remains true. Never fix a failed model merely by removing a minus sign.

Pause and explain

A downhill rough-slope calculation gives μ = −1/4. What is the appropriate conclusion?

Put the idea to work

Worked example

A 4 kg particle slides down a slope with sin θ = 3/5 and cos θ = 4/5. Its downward acceleration is 2 m s⁻² and only weight, reaction and sliding friction act. Find μ.

Show the worked solution
  1. Reaction is R = 40 × 4/5 = 32 N and downslope weight is 24 N.
  2. Downward Newton equation is 24 − 32μ = 4 × 2 = 8.
  3. Thus μ = 16/32 = 1/2; substituting gives the required 8 N resultant.

Answer μ = 1/2; friction 16 N up the plane.

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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 5 kg particle has a forward drive 30 N, an opposing resistance Q and acceleration 4 m s⁻² forward. Find Q.

Give me a hint

First calculate the resultant required by the mass and acceleration.

Compare my reasoning
  1. The required resultant is 5 × 4 = 20 N.
  2. The force equation is 30 − Q = 20.
  3. Therefore Q = 10 N, opposing motion as stated.

Q = 10 N.

Look for these in your work

  • Use ma for the resultant.
  • Retain the opposing sign for resistance.
2 · Independent

A particle of unknown mass m has a 42 N drive and 6 N opposing resistance, giving forward acceleration 3 m s⁻². Find m and test its sign.

Give me a hint

The driving and opposing forces determine the net force before dividing by acceleration.

Compare my reasoning
  1. The resultant is 42 − 6 = 36 N.
  2. Newton’s law gives 36 = 3m, so m = 12 kg.
  3. The positive mass is admissible and reproduces acceleration 36/12 = 3.

m = 12 kg.

Look for these in your work

  • Subtract the supplied resistance.
  • Check positive mass and substitute into the original equation.
3 · Transfer

A 4 kg particle slides downhill with sin θ = 3/5 and cos θ = 4/5. Only weight, reaction and resistive sliding friction act, but its reported downhill acceleration is 8 m s⁻². Infer μ and test the report.

Give me a hint

Compare the reported acceleration with the gravity-only downslope acceleration before accepting a parameter.

Compare my reasoning
  1. Reaction is 32 N and downslope weight is 24 N.
  2. The equation 24 − 32μ = 4 × 8 gives μ = −1/4.
  3. That coefficient is invalid for the stated resistive model; an additional driving force or corrected data would be needed.

Inferred μ = −1/4 is invalid; the report is incompatible with the stated model.

Look for these in your work

  • Quantify the incompatible parameter.
  • Keep the conclusion limited to the supplied force model rather than altering the sign.

Common mistakes

  • The unknown resistance is not usually the whole resultant.
  • Reject a negative coefficient in the stated resistive model.
  • A negative acceleration can still be physically valid.
Recall without your notes

What can you explain now?

Can you replace a calculated negative friction coefficient with its absolute value?

Compare with the explanation

No; that changes the equation and fails to reproduce the supplied acceleration.

The negative coefficient signals incompatibility with the stated force model.

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

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Make it stick

Infer the parameter from the full resultant equation, then substitute and check its physical constraints.

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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