Moments and Stability

after reading these cards hopefull you will be able to:

  • understand the concepts of mass, weight and centre of gravity
  • understand how levers work and what is meant by a " moment"
  • Understand what the "principle of moments" is
  • understand what is meant by the stability of an object.

thanks :)

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  • Created by: Mz
  • Created on: 31-03-10 08:03

Mass and Weight

  • Mass is the amount of matter in an object.
  • It is measured in kg.
  • Weight is a force.
  • The weight of an object depends on the mass of the object and gravity
  • weight is measured in newtons.


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Centre of gravity

The centre of Gravity of an object is the point through which all the object's mass or weight appears to act.

To find the centre of gravity for a rectangle or any regular shaped obect draw digonals across it - where they intersect is the centre of gravity.

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Levers and Moments

objective: to understand how levers work and what is ment by a "moment"

  • A lever is a device to help you complete a task.
  • all levers have a pivot and its position relative to the effort force and the load force determines how easily the lever enables us to carry out a task.
  • the closer the pivot is to the load force the less effort force is needed.

The moment / turning effect depends on three things:

  • The distance from the pivot
  • the effort force
  • the angle of force.


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Force is measured in Newtons

Distance is measured in cm, m

Moment is measured in Ncm, Nm

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Principle of moments

Objective: To understand what the "principle of moments" is.

For a system to balance, the clockwisew moments must be equal to the anticlockwise moment.

A weight of 10N is 2m from the pivot. It has a moment of 10 x 2 = 20Nm. Another weight weighs 20N and is 1m from the pivot. The moment is 20 x 1 = 20Nm, so the two weights are balanced (

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Objective: to understand what is menat bythe stability of an object.

Two features that control the angle of toppling are:

  • The height of the centre of gravity
  • the area of the base

A low centre of gravity and a large base will prevent toppling.

if the centre of gravity is higher up, when tilted, the weight will act outside the object sooner.

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thanx- excellent summary

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