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AP®PhysicsCollege BoardPhysics 1: Algebra-BasedStudy GuidesUnit 3: Work, Energy & PowerPowerCalculating Power

Calculating Power (College Board AP® Physics 1: Algebra-Based)

Study Guide

Leander Oates

Written by: Leander Oates

Reviewed by: Caroline Carroll

Calculating power

  • Power is defined as:

The rate at which energy changes with respect to time

  • The energy change could be:

    • a transfer of energy into or out of a system

    • a conversion from one energy type to another within a system

  • Average power is the amount of energy being transferred or converted, divided by the time taken for the transfer or conversion to occur

P subscript a v g end subscript space equals space fraction numerator increment E over denominator increment t end fraction

  • Where:

    • P subscript a v g end subscript = average power, measured in straight W

    • increment E = change in energy, measured in straight J

    • increment t = change in time, measured in straight s

  • The unit of power is the watt

    • 1 space straight W space equals space 1 space straight J divided by straight s

  • Since work done, W, is equal to the change in energy of an object or a system due to a force, average power can also be calculated using:

P subscript a v g end subscript space equals space fraction numerator W over denominator increment t end fraction

Worked Example

A warehouse worker is stacking boxes on a shelf. They lift a box of mass 18 space kg to a shelf of height 1.4 space straight m over a time interval of 3 space straight s. Which of the following is the average power output of the worker lifting the box?

A: 48 space straight W

B: 64 space straight W

C: 84 space straight W

D: 86 space straight W

The correct answer is C

Answer:

Step 1: List the known quantities

  • Mass, m space equals space 18 space kg

  • Height, increment y space equals space 1.4 space straight m

  • Change in time, increment t space equals space 3 space straight s

Step 2: State equation for average power

P subscript a v g end subscript space equals space fraction numerator increment E over denominator increment t end fraction

  • The energy transferred to the box is gravitational potential energy, therefore:

P subscript a v g end subscript space equals space fraction numerator m g increment y over denominator increment t end fraction

Step 3: Substitute in the known values to calculate

P subscript a v g end subscript space equals space fraction numerator 18 times 10 times 1.4 over denominator 3 end fraction

P subscript a v g end subscript space equals space 84 space straight W

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.