From Mechanics

Theory

Exercises

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Key Points

The moment of a force about the point \displaystyle O is the product of the force and the perpendicular distance to the line of action of the force from \displaystyle O.

\displaystyle \text{Moment }=Fd

\displaystyle \text{Moment }=Fd\sin \theta

Clockwise moments are negative.

Anti-clockwise moments are positive.

The resultant moment about \displaystyle O is the sum of the moments about \displaystyle O.

Example 13.1

Example 13.2

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 (Nm)
5N at \displaystyle O	\displaystyle 5\times 0=0
8 N	\displaystyle -8\times 1\textrm{.}2=-9\textrm{.}6
7 N	\displaystyle 7\times 0=0
6 N	\displaystyle -6\times 0\textrm{.}5=-3
5 N	\displaystyle 5\times 1\textrm{.}2=6
4 N	\displaystyle 4\times 0\textrm{.}5=2
Total Moment	\displaystyle 0-9\textrm{.}6+0-3+6+2=-4\textrm{.}6\text{ Nm}