Moments

1
Moments
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Learning Objectives

1. To fully understand what is meant by the moment
   of a force
2. To be able to solve problems involving moments

Most of this should be revision of Year 9 and GCSE
Book Reference Pages 97-98
3
Levers in everyday life
We are familiar with levers in everyday life,
they make our life easier.....
GIVE ME A PLACE TO STAND AND I WILL MOVE THE EARTH
4
Definitions
The moment of a force about any point is defined
as the force multiplied by the perpendicular
distance from the line of action to the point
Force f
Perpendicular distance d
Point
Moment f x d Units are Nm
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The Principle of Moments 1
If a body is acted upon by more than one force
and it is in equilibrium then the turning effects
of the forces must also balance out W1d1 W2d2
d1
d2
W1
W2
Pivot or Fulcrum
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The Principle of Moments 2
The sum of all the anticlockwise moments
The sum of all the clockwise moments


The sum of
The sum of
d1
d2
W1
W2
Pivot or Fulcrum
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The Principle of Moments 3
The sum of all the anticlockwise moments
The sum of all the clockwise moments

W1
W3
W2
W4
Pivot or Fulcrum
Even with more complex systems the same
fundamental rule applies
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Non-Perpendicular Forces
When the force is not perpendicular we use our
skills of resolving......
Force f
Perpendicular Force
90-?
Perpendicular distance d
?
Point
Moment f cos (90 - ?) x d
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Video Eureka! 12Warning When considering
masses, (kg) causing moments it is easy to forget
to convert the mass to a force (weight).
Numerically, it doesnt seem to matter but you
will lose marks since it shows poor conceptual
understanding
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Summary The principle of moments changes
little from KS3 to KS4 to KS5
Our skills at resolving forces into components
can be utilised to deal with forces which are not
perpendicular to the distance from the pivot