Learn in your language
Skip to content
A Level · Cambridge (CIE) · 9709

Distinguish average power from instantaneous driving power

Convert watts and kilowatts, use work divided by time for an average and P = Fv for a force along the current motion.

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

  • Calculate work and distinguish an interval average from an instantaneous value.
  • Convert joules, watts and kilowatts.

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

Quick readiness check

What is average power when 12 000 J of work is done in 8 s?

What you will learn

  • Calculate average power over an elapsed interval.
  • Use instantaneous driving power, force and speed with compatible units.
At constant driving power 18 kW, force is 1500 N at speed 12 m s⁻¹ and 750 N at speed 24 m s⁻¹. The positive-speed curve is F = 18000/v. The two marked rectangles have equal force-times-speed products, 18000 W, although their forces differ. The curve is displayed only for positive speeds 8 to 28 m s⁻¹; it does not define a finite force at zero speed.Constant power gives variable forceP = 18 kW; F = 18000/v81220242875015002000(12, 1500)(24, 750)Speed v / m s⁻¹Force / NBoth Fv products are 18000 WPositive speeds only; no v = 0 value
At constant driving power 18 kW, force is 1500 N at speed 12 m s⁻¹ and 750 N at speed 24 m s⁻¹. The positive-speed curve is F = 18000/v. The two marked rectangles have equal force-times-speed products, 18000 W, although their forces differ. The curve is displayed only for positive speeds 8 to 28 m s⁻¹; it does not define a finite force at zero speed.

Power measures a rate of doing work

Average power over an interval is total work divided by elapsed time. Doing 12 000 J of work in 8 s gives average power 1500 W = 1.5 kW. One watt is one joule per second; 1 kW is 1000 W. The average need not equal the power at every instant, so it cannot by itself supply a unique instantaneous drive force when speed changes.

For a force acting along the motion, instantaneous power is P = Fv, with v the current speed in that direction. A drive force 400 N at speed 15 m s⁻¹ supplies power 6000 W = 6 kW. Conversely, a drive supplying 18 kW at speed 12 m s⁻¹ has force F = 18 000/12 = 1500 N. Convert kilowatts before dividing; force is in newtons, not watts.

Driving power is not necessarily net kinetic-energy rate

At constant speed, a vehicle can require positive driving power to balance resistance and, on a hill, gravitational opposition. On a level road against 600 N resistance at speed 20 m s⁻¹, the drive needs 600 N and power 12 kW even though kinetic energy is unchanged. The supplied power is transferred through resisting work, rather than being entirely stored in speed.

A constant driving power gives F = P/v for positive speed, so drive force decreases as speed rises. At v = 0 this ideal relation cannot give a finite starting force; a start-from-rest model needs a separately supplied force or power regime. Do not divide by zero or assume constant acceleration for an entire journey merely because power is constant. For an angled force, use its along-motion component rather than applying the parallel-force formula to its full magnitude.

Pause and explain

At constant drive power and positive speed, what happens to F = P/v when speed doubles?

Put the idea to work

Worked example

A drive supplies 18 kW while its vehicle moves at 12 m s⁻¹ in the force direction. Find the driving force.

Show the worked solution
  1. Convert 18 kW to 18 000 W.
  2. Use P = Fv, so F = P/v = 18 000/12.
  3. The force is 1500 N at this instant; it need not stay constant at another speed.

Answer Instantaneous driving force 1500 N.

Try a topic test

Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.

Test this topic →

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 force does 12 000 J of work during an 8 s interval. Find average power in watts and kilowatts.

Give me a hint

Divide work by elapsed time, then convert watts to kilowatts.

Compare my reasoning
  1. Average power is 12 000/8 = 1500 W.
  2. Since 1000 W = 1 kW, this is 1.5 kW.
  3. The result is an interval average, not automatically the power at every instant.

Average power 1500 W = 1.5 kW.

Look for these in your work

  • Use work divided by time.
  • Distinguish the average from an unprovided instantaneous rate.
2 · Independent

A drive force 400 N acts along motion at speed 15 m s⁻¹. Find instantaneous power in watts and kilowatts.

Give me a hint

The force and velocity are along the same direction, so use P = Fv.

Compare my reasoning
  1. Power is 400 × 15 = 6000 W.
  2. This equals 6 kW.
  3. The result applies at the stated instantaneous speed.

Instantaneous power 6000 W = 6 kW.

Look for these in your work

  • Use the along-motion force.
  • Convert the final watt value correctly.
3 · Transfer

A drive supplies constant 18 kW while moving at positive speeds 12 and 24 m s⁻¹. Find the driving force at each speed and explain whether the same formula gives a finite force at rest.

Give me a hint

Convert the power once and divide it separately by each positive speed.

Compare my reasoning
  1. At 12, F = 18 000/12 = 1500 N.
  2. At 24, F = 18 000/24 = 750 N, half the force.
  3. At zero speed P/v is undefined, so a separate starting-force regime is required rather than a finite value from this formula.

Forces 1500 N and 750 N; the zero-speed case needs a different starting model.

Look for these in your work

  • Use instantaneous speeds rather than an unsupported average.
  • State the positive-speed limitation of the ideal constant-power relation.

Common mistakes

  • Convert kW to W before using P = Fv.
  • Average power need not be an instantaneous value.
  • Positive drive power can be needed at constant speed.
Recall without your notes

What can you explain now?

Can constant positive power P be used as F = P/v at zero speed to predict a finite starting force?

Compare with the explanation

No; the ideal formula is singular at v = 0 and needs a different starting model.

Its valid positive-speed interval must be stated instead of inventing a finite force from division by zero.

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

Your review choice appears on Today. Sign in to sync it across devices.

Make it stick

Use W/time for an average and Fv for instantaneous parallel driving power, with SI units and a stated valid speed interval.

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.

Keep learning Mathematics

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.

StudyForA · Learn with understanding

Original teaching content and links to official sources. Check your exam year and qualification before using a resource.