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A LevelPhysicsCambridge (CIE)Revision Notes12. Motion in a CircleCentripetal AccelerationCalculating Centripetal Force

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First teaching 2023

First exams 2025

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Calculating Centripetal Force (Cambridge (CIE) A Level Physics) : Revision Note

Leander Oates

Written by: Leander Oates

Reviewed by: Caroline Carroll

Updated on

Calculating centripetal force

F space equals fraction numerator space m v squared over denominator r end fraction

  • Where:

    • F = magnitude of the centripetal force, measured in newtons (N)

    • m = mass of object, measured in kilograms (kg)

    • v = velocity, measured in meters per second (m s-1)

    • r = radius of circular path, measured in metres (m)

  • Another form of the equation can be written using the equation linking linear and angular velocity:

v space equals space r omega

  • Where

    • omega = angular velocity measured in radians per second (rad s-1)

  • Substituting into the first centripetal force equation

F space equals space fraction numerator m open parentheses r omega close parentheses squared over denominator r end fraction

  • Simplify the equation

F space equals space fraction numerator m r squared omega squared over denominator r end fraction space equals space m r omega squared

Centripetal force in circular motion

Centripetal force diagram, downloadable AS & A Level Physics revision notes

Centripetal force is always perpendicular to the direction of travel

Worked Example

A bucket of mass 8.0 kg is filled with water is attached to a string of length 0.5 m.

What is the minimum speed the bucket must have at the top of the circle so no water spills out?

WE - Centripetal force question image, downloadable AS & A Level Physics revision notes

Answer:

Step 1: Draw the forces on the bucket at the top

Step 2: Calculate the centripetal force

  • The weight of the bucket = mg

  • This is equal to the centripetal force since it is directed towards the centre of the circle

m g space equals fraction numerator space m v squared over denominator r end fraction

Step 3: Rearrange for velocity v

  • m cancels from both sides

v space equals space square root of g r end root

Step 4: Substitute in values

v space equals space square root of 9.81 space cross times space 0.5 end root space equals space 2.21 space straight m space straight s to the power of negative 1 end exponent

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Senior Physics Content Creator

    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

    Reviewer: 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.