13. Moments
From Mechanics
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Revision as of 16:13, 17 September 2009
Theory | Exercises |
Key Points
The moment of the force about the point O is the product of the force and the perpendicular distance to the line of action of the force from O.
Clockwise moments are negative.
Anti-clockwise moments are positive.
Example 11.1
Find the moment of each force shown below about the point O.
(a) (b)
Solution
(a) (b)
0
8=16
1
2=14
4
Moment is -16 Nm Moment is 14.4 Nm
Example 13.2 Find the moment of each force shown below about the point O. (a) (b)
Solution
(a) (b)
3sin60
=104
2sin20
=68
4
Moment is 104 Nm (to 3 sf) Moment is 68.4 Nm (to 3 sf)
Example 13.3 For the rectangular lamina shown below, find the total moment of the forces acting, about the corner marked O.
Solution
Force Moment
5N at O
0=0
8 N
1
2=−9
6
7 N
0=0
6 N
0
5=−3
5 N
1
2=6
4 N
0
5=2
Total Moment
6+0−3+6+2=−4
6 Nm