1. Moments

2. Introduction For stability or equilibrium we have already seen that:
For rotational forces this means that the effect of the forces rotating the object clockwise must equal the forces rotating the object in an anticlockwise direction. A MOMENT is the measure of the turning effect of a force.

3. What is a Moment? A moment is defined as:
MOMENT (M) =
force (f) x perpendicular distance from a pivot (d)
So the units of a moment are:
Newton metres or Nm

4. There are two ways of increasing the size of a moment……..
Increase the size of the force
Increase the distance from the pivot
This can be seen in several everyday situations:
A long spanner undoes a nut easier then a short one
Its hard to open a door by pushing near the hinge
a large weight can be lifted using a large lever
A crane has a limited working radius

5. Increasing the moment….

6. For equilibrium (balance):
The sum of the clockwise moments must be equal to the sum of the anticlockwise moments
(moments may be taken about any point on a structure)
This is known as ‘The principle of Moments’

7. For Example…..

8. Example 2 For Balance clockwise moments = Anticlockwise moments
Unknown weight y=?
Clockwise moments about fulcrum=
yx0.1
+5x0.4=0.1y+2
Anticlockwise moments about fulcrum=
20x0.3=6
For Balance clockwise moments = Anticlockwise moments
0.1y+2.0= y= y=40N

9. Example 3 What are the sizes of the reactions X and Y at the trestles?
Hints:
Take moments about any point
Total Upwards forces must equal total downwards forces
Moments through the point chosen as fulcrum=0
Answers: Y=240N X=360N