Edexcel International A Level Maths: Mechanics 1

Revision Notes

4.1.1 Moments Diagrams

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Moments Diagrams

When an object is modelled as a particle we do not need to consider the rotation of the object due to forces. When we model an object as a rigid body then we need to consider the rotation of the object.

What is a moment?

  • The moment of a force is the measure of its ability to cause a body to rotate about a specific point (usually called the pivot)
  • The units for moments are newton metres (N m)
  • A force can cause a body to rotate clockwise or anti-clockwise
  • The moment of a force about a point is calculated by multiplying the magnitude of the force by the perpendicular distance from the line of action of the force to the point
    • We can choose the positive direction to be clockwise or anti-clockwise and the negative direction is the opposite
    • A zero moment will not cause a body to rotate

  • The moment of a force about a point is zero if the line of action of the force goes through the point
  • If the force is not acting perpendicular to the distance given you will need to use right-angled trigonometry to find the perpendicular distance

4.1.1 Moments Diagrams Diagram 1, downloadable Edexcel A Level Mechanics revision notes

How do you decide if a moment is clockwise or anti-clockwise?

  •  A clockwise moment will cause a body to rotate in the clockwise direction
  • To visualise this:
    • Imagine that the perpendicular line from the point to the force is the radius of a circle
    • The direction of the force tells you which way the circle goes round

4.1.1 Moments Diagrams Diagram 2, downloadable Edexcel A Level Mechanics revision notes

What is the resultant moment?

  • The resultant moment is the sum of all the moments acting on a body (both positive and negative)
  • To find the resultant moment:
    • Define the positive direction (clockwise or anti-clockwise)
    • Add together all the moments in that direction
    • Subtract all the moments in the opposite direction
    • The overall value is the resultant moment

  • The sign of the resultant moment indicates which direction the body will rotate
    • Positive will be the direction chosen as positive
    • Negative will be the opposite direction

Worked example

4.1.1_WE_Moments_1

4-1-1-moments-diagrams-example-solution

Exam Tip

  • Always define a positive direction as either clockwise or anti-clockwise.
  • It is best to state the magnitude of a moment and write its direction when giving a final answer.

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Dan

Author: Dan

Dan graduated from the University of Oxford with a First class degree in mathematics. As well as teaching maths for over 8 years, Dan has marked a range of exams for Edexcel, tutored students and taught A Level Accounting. Dan has a keen interest in statistics and probability and their real-life applications.