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A Level · Cambridge (CIE) · 9709

Include friction and driving work on a rough slope

Calculate path-dependent friction work, height-dependent potential change and the reaction change caused by an angled pull.

Mechanics · Energy, work and power · 4.5
Exam scope and assessment notes

Mechanics (Paper 4, AS & A Level) · Mathematics 9709, 2026–2027, section 4.5. Use g = 10 m s⁻² throughout these problems. Masses are positive and constant. Work by a constant force is Fd cos θ, with θ between force and displacement; scalar-product notation is not required. Kinetic energy is ½mv² and gravitational potential-energy change is mgΔh. State the system and count gravitational work or potential-energy change once, not both. Mechanical energy is conserved only under the stated loss-free model; friction and supplied resistance transfer mechanical energy to other forms. Smooth curved-slide examples use overall energy changes and assume contact throughout, rather than requiring a normal-reaction calculation. Average power is work divided by elapsed time; P = Fv applies to a force along the motion. Constant driving power generally gives variable force and acceleration; instantaneous calculations do not justify constant-acceleration formulae over a whole journey. Check string, contact and motion-state changes before continuing a model. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Resolve weight and an applied force relative to a plane.
  • Use μR for sliding friction with its direction opposite motion.

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

Quick readiness check

For a constant sliding friction f over travel distance s, what work enters the particle’s energy equation?

What you will learn

  • Use a mechanical-energy balance with supplied friction and drive work.
  • Find reaction before friction for an angled pull and check contact and stopping.
The 5 kg particle descends 4 m on a rough plane with sine 0.6, cosine 0.8 and coefficient 0.25. Its vertical drop is 2.4 m, releasing 120 J. Friction transfers 40 J to other forms, leaving kinetic energy 80 J and speed 4√2 m s⁻¹. Bar widths are proportional to positive energy amounts. Mechanical energy decreases by the friction loss; the total energy is not destroyed.A rough slope transfers energyPath 4 m; vertical drop 2.4 mGravitational energy released120 JFinal K + friction loss80 J40 JFinal KFriction loss120 − 40 = 80 J → speed 4√2Energy transferred, not destroyed
The 5 kg particle descends 4 m on a rough plane with sine 0.6, cosine 0.8 and coefficient 0.25. Its vertical drop is 2.4 m, releasing 120 J. Friction transfers 40 J to other forms, leaving kinetic energy 80 J and speed 4√2 m s⁻¹. Bar widths are proportional to positive energy amounts. Mechanical energy decreases by the friction loss; the total energy is not destroyed.

Friction work uses path distance while gravity uses height

A 5 kg particle slides down a plane with sin θ = 3/5, cos θ = 4/5 and μ = 1/4. Reaction is 40 N and sliding friction is 10 N up the plane. Over distance 4 m, vertical drop is 2.4 m and potential-energy decrease is 120 J. Friction transfers 40 J out of mechanical energy, so a particle starting from rest ends with K = 80 J and speed 4√2 m s⁻¹.

Equivalently, total work on the particle is gravitational work +120 J plus friction work −40 J, giving ΔK = +80 J. Use either this form or K + U with non-gravitational work, not both gravity contributions together. On the upward leg friction and gravity both remove kinetic energy: for that particle projected up at speed 8, initial K is 160 J and the energy cost per metre is 30 + 10 = 40 J, giving stopping distance 4 m.

An angled applied force can change the loss

For a 5 kg particle pulled up a 30° slope by 40 N at angle φ above the plane, with sin φ = 3/5 and cos φ = 4/5, the normal equation is R = 50 cos 30° − 40 sin φ = 25√3 − 24 N, about 19.30 N. At μ = 1/5 the friction magnitude is (25√3 − 24)/5, about 3.86 N. The along-plane drive is 32 N and downslope weight component is 25 N, so the net uphill work per metre is 32 − 25 − friction.

If this particle starts from rest and moves 3 m up, final K is 3[7 − (25√3 − 24)/5] = 177/5 − 15√3 J, about 9.42 J, giving speed about 1.94 m s⁻¹. The positive reaction permits contact and the uphill drive exceeds weight plus maximum friction, supporting the stated start. A pull large enough to make R negative would invalidate this contact calculation. At a stopping event, test static balance before assuming a return.

Pause and explain

An upward angled pull reduces a slope particle’s reaction. What must be done before calculating sliding friction?

Put the idea to work

Worked example

A 5 kg particle starts sliding down a rough slope from rest and travels 4 m. Given sin θ = 3/5, cos θ = 4/5 and μ = 1/4, find its final speed.

Show the worked solution
  1. R = 40 N and friction is 10 N, so friction work is −40 J.
  2. The vertical drop is 2.4 m, so potential-energy decrease is 120 J.
  3. Final K is 120 − 40 = 80 J; ½ × 5 × v² = 80 gives speed 4√2 m s⁻¹.

Answer Final K = 80 J; speed 4√2 m s⁻¹.

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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 starts from rest and slides 4 m down a rough slope with sin θ = 3/5, cos θ = 4/5 and μ = 1/4. Find final kinetic energy and speed.

Give me a hint

Subtract friction’s path loss from the height-based gravitational release.

Compare my reasoning
  1. Reaction is 40 N, friction is 10 N and friction loss over 4 m is 40 J.
  2. Vertical drop is 4 × 3/5 = 2.4 m, releasing 120 J gravitational energy.
  3. K = 80 J, so v² = 32 and speed is 4√2 m s⁻¹.

Final K = 80 J; speed 4√2 m s⁻¹.

Look for these in your work

  • Use different geometric distances for gravity and friction.
  • Retain the friction loss with the correct sign.
2 · Independent

A 5 kg particle is projected up the same long rough slope at speed 8 m s⁻¹. Find the distance to first rest, and check whether it restarts downhill using the same coefficient for the static limit.

Give me a hint

During upward sliding, both gravity and friction remove kinetic energy.

Compare my reasoning
  1. Initial K is ½ × 5 × 64 = 160 J.
  2. The energy cost per metre is 50(3/5) + (1/4)50(4/5) = 40 J/m, so stopping distance is 4 m.
  3. At rest, the 30 N downslope weight component exceeds the 10 N static limit, so it restarts downhill under the stated model.

Stops after 4 m; downhill restart occurs because 30 N exceeds the 10 N static limit.

Look for these in your work

  • Count uphill potential gain and friction loss together.
  • Test the static state at first rest before asserting restart.
3 · Transfer

A 5 kg particle is pulled up a 30° plane by 40 N at angle φ above the plane, with sin φ = 3/5 and cos φ = 4/5. With μ = 1/5, it starts from rest and travels 3 m up. Find reaction, final kinetic energy and speed, checking contact.

Give me a hint

The pull’s normal component changes the friction before you form the along-path energy balance.

Compare my reasoning
  1. R = 25√3 − 24 N, approximately 19.30 N, so contact is admissible; friction is R/5.
  2. Driving work is 32 × 3 = 96 J, potential gain is 25 × 3 = 75 J and friction loss is 3R/5.
  3. Final K = 177/5 − 15√3 J, approximately 9.42 J, and v = √(2K/5), approximately 1.94 m s⁻¹.

R = 25√3 − 24 N; K = 177/5 − 15√3 J ≈ 9.42 J; speed ≈ 1.94 m s⁻¹.

Look for these in your work

  • Check positive reaction and uphill drive before applying the sliding model.
  • Include drive work, potential gain and reaction-dependent friction exactly once.

Common mistakes

  • Friction uses path distance; mgΔh uses vertical height.
  • An angled pull changes R and therefore μR.
  • Do not claim mechanical-energy conservation when friction does work.
Recall without your notes

What can you explain now?

Does a rough slope generally conserve K + U for the moving particle?

Compare with the explanation

No; friction transfers mechanical energy to other forms.

Include its negative work and any supplied driving work in the balance.

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Make it stick

Resolve contact first, then balance drive work against kinetic change, vertical potential change and friction’s path loss.

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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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