Topic Five (1.5): Moments

Introduction

This is probably one of the most challenging topics in A Level Physics and is worth spending plenty of time on. It all hinges around ‘see-saw’ style problems, and you will be required to find missing forces or distances in balanced situations. To tell if an object is stable, force equilibrium (Topic 1.4) is not enough. We must also consider turning effects (moments) and whether these balance too. This topic gives the precise definition of a moment as force multiplied by perpendicular distance from the pivot point, and how to use the principle of moments to check for moment equilibrium. It also defines the centre of mass, the point where we can consider all an object’s mass is in effect concentrated. This makes it simple to work out how an object will move or rotate.

Reading

Edexcel International AS/A Level Physics Student Book 1 pages 17 to 19

Tasks

Complete the following:

  • Read pages 17 to 19 of the textbook.

WATCH –The Law of the Lever (Niall Murphy) (YouTube) Links to an external site.

WATCH – The Principle of Moments (Physics Online) Links to an external site.

SIMULATION – Level Principle (Walter Fendt) Links to an external site.

COMPLETE – Gameboard: ‘Moments’ (Isaac Physics) Links to an external site.

WATCH – Centre of Mass and Gravity (Physics Online) Links to an external site. Note how the centre of mass is an ‘effectively equivalent’ situation. Mass is spread throughout an object, but if we replace all these individual mass elements with a total mass concentrated at the ‘centre of mass’, it gives the exact same effects. Unless in a very non-uniform gravity field (where some masses are pulled more strongly than others), centre of mass is just the same as centre of gravity.

  • Read the Practical Skills box on page 19.

WATCH – Centre of Mass Experiment (Chris Gozzard) Links to an external site.

  • Experiment finding the centre of mass of an irregular object (copy the Chris Gozzard video; cut your own irregular shape from cardboard).
  • Complete the questions on page 19. You will find the answers in General Resources under the heading Textbook Answers. Download Textbook Answers.

Top Tips

Make sure that you understand the following key points:

  • Take care to be consistent with signs – all clockwise rotations are positive and anticlockwise are negative (or vice versa). Then for moment equilibrium, all moments will sum to zero.
  • The clockwise moments and the anticlockwise moments must all be taken about the same pivot point.
  • Make sure you use the perpendicular distance from the force to the pivot.
  • When balancing moments applied to an object, do not forget the moment due to the object’s own weight.
  • If any object hangs freely under gravity, its centre of gravity will lie directly below the suspension point.

Key Terms

Add the following key terms with definitions to your word list:

  • Moment
  • Equilibrium
  • Principle of moments
  • Centre of mass
  • Centre of gravity