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A Level · Cambridge (CIE) · 9709

Resolve forces and calculate a resultant with its direction

Calculate signed perpendicular components, add them and recover a resultant’s magnitude and direction in the correct quadrant.

Mechanics · Forces and equilibrium · 4.1
Exam scope and assessment notes

Mechanics (Paper 4, AS & A Level) · Mathematics 9709, 2026–2027, section 4.1. Paper 1 algebra and trigonometry are assumed. Bodies are modelled as particles: forces act at one point and rotational effects are not considered. Resolve numerically; a scale drawing is not a substitute for calculation. Force diagrams are qualitative unless stated otherwise. Where mass is supplied in this sequence, use the stated g = 10 m s⁻². Check non-negative tension and contact reaction and use F ≤ μR except at limiting friction. Written practice is self-checked; human subject-teacher review remains pending.

Before you begin

  • Use sine, cosine and Pythagoras.
  • Locate signed components in Cartesian quadrants.

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

Quick readiness check

For P at θ above the horizontal, what is its horizontal component?

What you will learn

  • Resolve a force relative to explicitly chosen axes.
  • Calculate the magnitude and directed angle of a resultant from its components.
Add signed force components8 N right + 6 N up8 N6 N10 N36.9°Equal force scales on both axes
The two perpendicular components are 8 N right and 6 N up, shown with equal force scales along both axes. The blue diagonal is the combined resultant, of magnitude 10 N and angle arctan(6/8), approximately 36.9° above the horizontal. The dashed rectangle shows component projection. The resultant replaces these forces for calculating their combined effect; it is not a third external force to add to them. Calculations determine the values, rather than a scale-drawing estimate.

Components depend on where the angle is measured

A force P at angle θ above the positive horizontal has rightward component Pcos θ and upward component Psin θ. If θ is measured from the vertical instead, the roles of sine and cosine swap. Components can be negative when a force points against a chosen positive direction. State the axes and angle reference before calculating; the component adjacent to the given angle uses cosine.

For two perpendicular forces 8 N right and 6 N up, the resultant components are 8 and 6. Its magnitude is √(8² + 6²) = 10 N, and its angle above the rightward horizontal is arctan(6/8) ≈ 36.9°. Its magnitude is not the sum 14 N. A force triangle can illustrate the geometry, but the numerical calculation is required rather than an approximate scale measurement.

Add signed components before finding the resultant

For several forces, add all components along each axis to get H and V. Then resultant magnitude is √(H² + V²), with direction set by their signs. For 10 N east, 6 N west and 8 N south, H = 4 and V = −8; the resultant is 4√5 N, directed 63.4° below east. An unsigned inverse tangent alone would not establish the quadrant.

If H = V = 0, the resultant is zero and has no direction angle. Forces can cancel without each being zero. A resultant acts as the combined force in the particle model; it is not an extra force to add to the original force diagram. To balance a non-zero resultant, apply an equal-magnitude opposite-direction force, called an equilibrant.

Pause and explain

What is the resultant magnitude of perpendicular 8 N and 6 N forces?

Put the idea to work

Worked example

Find the resultant of 8 N right and 6 N up, giving magnitude and direction.

Show the worked solution
  1. The resultant’s perpendicular components are H = 8 N and V = 6 N.
  2. Its magnitude is √(64 + 36) = 10 N.
  3. Its quadrant I angle is arctan(6/8) ≈ 36.9° above the rightward horizontal.

Answer 10 N, 36.9° above the rightward horizontal.

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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

Resolve a 12 N force at 60° above the rightward horizontal into horizontal and vertical components.

Give me a hint

Use the components adjacent and opposite to the specified horizontal angle.

Compare my reasoning
  1. The rightward component is 12cos60° = 6 N.
  2. The upward component is 12sin60° = 6√3 N.
  3. Their squared magnitudes total 36 + 108 = 144, recovering the force magnitude 12 N.

6 N right and 6√3 N up.

Look for these in your work

  • Use the stated angle reference.
  • Check the recovered magnitude from both components.
2 · Independent

Find the resultant of forces 10 N east, 6 N west and 8 N south, including its direction.

Give me a hint

Take east and north as positive and add signed components first.

Compare my reasoning
  1. H = 10 − 6 = 4 N and V = −8 N.
  2. The magnitude is √(16 + 64) = 4√5 N ≈ 8.94 N.
  3. It points southeast, at arctan(8/4) ≈ 63.4° below east.

4√5 N ≈ 8.94 N, 63.4° below east.

Look for these in your work

  • Keep the west and south signs when summing.
  • State a direction consistent with both component signs.
3 · Transfer

A 30 N force points at an acute angle above the rightward horizontal and has upward component 18 N. Find its horizontal component and the resultant after adding 16 N right and 18 N down.

Give me a hint

The acute-angle condition fixes the positive horizontal component.

Compare my reasoning
  1. The horizontal component is √(30² − 18²) = 24 N right.
  2. The added horizontal force gives H = 24 + 16 = 40 N.
  3. The vertical forces cancel: V = 18 − 18 = 0, giving a 40 N rightward resultant.

Original horizontal component 24 N; combined resultant 40 N right.

Look for these in your work

  • Use the given acute direction to select the component sign.
  • Combine all signed components before finding the resultant.

Common mistakes

  • Identify the axis from which the angle is measured before choosing sine or cosine.
  • Add signed components, not force magnitudes.
  • A resultant replaces the original forces for balance; it is not another external force.
Recall without your notes

What can you explain now?

What is the equilibrant of a 10 N resultant directed rightwards?

Compare with the explanation

10 N leftwards.

It has equal magnitude and opposite direction, making the total resultant zero.

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

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Make it stick

Resolve relative to labelled axes, add signed components and calculate the resultant’s magnitude, quadrant and equilibrant.

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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