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13. Moments

From Mechanics

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|8 N
|8 N
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|valign="top"| <math>-8\times 1.2=-9.6</math>
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|valign="top"| <math>-8\times 1\textrm{.}2=-9\textrm{.}6</math>
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|6 N
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| valign="top"| <math>-6\times 0.5=-3</math>
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| valign="top"| <math>-6\times 0\textrm{.}5=-3</math>
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|5 N
|5 N
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| valign="top"| <math>5\times 1.2=6</math>
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| valign="top"| <math>5\times 1\textrm{.}2=6</math>
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|4 N
|4 N
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| valign="top"| <math>4\times 0.5=2</math>
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| valign="top"| <math>4\times 0\textrm{.}5=2</math>
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|Total Moment
|Total Moment
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| valign="top"| <math>0-9.6+0-3+6+2=-4.6\text{ Nm}</math>
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| valign="top"| <math>0-9\textrm{.}6+0-3+6+2=-4\textrm{.}6\text{ Nm}</math>
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Revision as of 09:39, 23 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.


Image:T13.1.GIF

Moment =Fd


Image:T13.2.GIF

Moment =Fdsin


Clockwise moments are negative. Anti-clockwise moments are positive.


Example 13.1

Image:ex13.1whole.GIF



Example 13.2

Image:ex13.2.GIF


Example 13.3

For the rectangular lamina shown below, find the total moment of the forces acting, about the corner marked O.


Image:ex13.3.GIF

Solution

Force Moment
5N at O 50=0
8 N 81.2=9.6
7 N 70=0
6 N 60.5=3
5 N 51.2=6
4 N 40.5=2
Total Moment 09.6+03+6+2=4.6 Nm