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.
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.
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?
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
- Convert 18 kW to 18 000 W.
- Use P = Fv, so F = P/v = 18 000/12.
- 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.
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
- Average power is 12 000/8 = 1500 W.
- Since 1000 W = 1 kW, this is 1.5 kW.
- 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.
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
- Power is 400 × 15 = 6000 W.
- This equals 6 kW.
- 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.
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
- At 12, F = 18 000/12 = 1500 N.
- At 24, F = 18 000/24 = 750 N, half the force.
- 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.
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.