Your Equation

Library / Tools / A-level physics equation calculator / Power from force and speed

Power from force and speed calculator

A-level Energy and power

The power needed to move at constant speed against a constant force.

P = F v

This free calculator is for A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

10000 W

Working

  1. Write the equation
    P = F v
  2. Substitute the values
    P = 400 × 25
  3. Calculate
    P = 10000 W
  4. Give the answer to 3 significant figures
    P = 10000 W
    In standard form: 1.00 × 104 W

Worked example

Find P, the power. Given: driving force F = 400 N; speed v = 25.0 m/s.

  1. Write the equation
    P = F v
  2. Substitute the values
    P = 400 × 25
  3. Calculate
    P = 10000 W
  4. Give the answer to 3 significant figures
    P = 10000 W
    In standard form: 1.00 × 104 W

Symbols and units

SymbolQuantitySI unit
PPowerW
FDriving forceN
vSpeedm/s

Power from force and speed: rearranged

  • To find power: P = F v
  • To find driving force: F = P / v
  • To find speed: v = P / F

Exam tip

At constant speed the driving force equals the resistive force. Use that to find F.

Common questions

What is the formula for power from force and speed?

The equation is P = F v, where P is the power (W); F is the driving force (N); v is the speed (m/s).

What units should I use for power from force and speed?

In SI units: power in W, driving force in N, speed in m/s. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange P = F v?

Rearranging gives these forms: to find power: P = F v. to find driving force: F = P / v. to find speed: v = P / F.

Practise power from force and speed questions

Press the button for a question with random numbers. Work it out on paper first, then type your answer.

Related

Read the full explanation on the Mechanical power formula page. More energy and power equations: Work done, Work done by an angled force, Elastic potential energy, Kinetic energy, Gravitational potential energy, Power, Energy transferred, Efficiency. See the full list on the a-level physics equation calculator page.