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2.4. REQUIREMENT OF STABLE EQUILIBRIUM

For stable equilibrium of a body, the algebraic sum of all the external forces should be zero and also the algebraic sum of moments of all the external forces about any point in their plane is zero. Mathematically, it is expressed by the equations :

ƩF = 0

ƩM = 0

The sign Ʃ is known as sigma which is a Greek letter. This sign represents the algebraic sum of forces or moments.

For two-dimensional bodies, the forces are generally resolved into horizontal and vertical components. Hence we have

ƩFx = 0

and    ƩFy = 0

where ƩFx = Algebraic sum of all horizontal components

and ƩFy = Algebraic sum of all vertical components

Similarly for moments, we have

ƩMx = 0    and              ƩMy = 0