Page 27 - mechanics

Chapter – 5 Equilibrium of rigid body & Reactions of supports

the body. Thus, when a support prevents translation, it exerts a
force in opposite direction and when it prevents rotation it
exerts a couple moment in opposite direction. These forces and
couple moments are what we call reactions of supports.

The reactions of a support can be determined by defining
the motion it prevents. You should also note that any support
has a maximum allowable load to support and beyond this
value it cracks. For example, while you can not move or rotate
a fixed street light pole, a hurricane sweeps it out of its place.

5.2 Equilibrium of Two-Dimensional Rigid Body
We saw in the preceding chapter that the external forces

acting on a rigid body can be reduced to a force-couple system
at some arbitrary point O. When the force and the couple are
both equal to zero, the external forces form a system equivalent
to zero, and the rigid body is said to be in equilibrium. The
necessary and sufficient conditions for the equilibrium of a
rigid body, therefore, can be obtained by the master equations

F = 0, MO = 0

where O is any point in the plane of the body. Resolving each
force and each moment into its rectangular components, the
necessary and sufficient conditions for the equilibrium of a

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