Revision NotesExam QuestionsPast Papers
ASPhysicsCIERevision Notes4. Forces, Density & Pressure4.1 Turning Effects of ForcesMoments

Moments (CIE AS Physics)

Revision Note

Leander

Author

Leander

Last updated

What is a moment?

  • A moment is the turning effect of a force
  • Moments occur when forces cause objects to rotate about a pivot

  • The moment of a force is given by the equation:

moment space equals space F d

  • Where:
    • moment = turning effect in newton metres (N m)
    • F = force in newtons (N)
    • d = perpendicular distance from the pivot in metres (m)

  • The force may not always be applied perpendicularly to the pivot
  • However, the distance used in the equation must be the perpendicular distance from the pivot
  • In these cases, the component of the distance that is perpendicular to the pivot must be used

Force applied at an angle to the pivot

Perpendicular-distance, downloadable AS & A Level Physics revision notes

When the force is applied at an angle to the pivot, the component of the distance that is perpendicular must be used

  • An example of moments in everyday life is opening a door
    • The door handle is placed on the opposite side of the door to the hinge (the hinge is the pivot)
    • This maximises the distance for a given force
    • Which provides a greater moment (turning effect)
      • This makes it easier (requires less force) to push or pull the door open

Worked example

A uniform metre ruler is pivoted at the 50 cm mark.

A 0.5 kg weight is suspended at the 80 cm mark, causing the ruler to rotate about the pivot.

Calculate the turning moment about the pivot. Assume the weight of the ruler is negligible. 

WE - Mass on ruler question image, downloadable AS & A Level Physics revision notes

Answer: 

Step 1: List the known quantities and convert to SI units

  • Pivot at 0.50 m
  • Mass, m = 0.5 kg
  • Force applied at 0.80 m

Step 2: State the equation for moment

moment space equals space F d

Step 3: Identify the force applied

  • The only force acting is the force of weight on the mass

W space equals space m g space

W space equals space 0.5 space cross times space 9.81

W space equals space 4.905 space straight N

Step 4: Identify the perpendicular distance from the pivot

d space equals space 0.80 space minus space 0.50 space equals space 0.30 space straight m

Step 5: Substitute the known values into the moment equation to calculate

moment space equals space 4.905 space cross times space 0.30

moment space equals space 1.4715 space straight N space straight m

moment space equals space 1.5 space straight N space straight m space open parentheses 2 space straight s. straight f. close parentheses

Examiner Tip

It is better to only round your answer right at the end of your calculation. Always work with at least one, preferably two more significant figures than your final answer requires. This will help you to avoid compound rounding errors that could lose you marks. 

Sketching a quick diagram of all the forces acting on the object will help you visualise which forces are perpendicular to the distance from the pivot. Not all forces will provide a turning effect and it is common for questions to provide more forces than required to increase the level of difficulty of the question.

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:Centre of GravityNext:Turning Effects of Forces
    Leander

    Author: Leander

    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.