Strength of Materials
A heavy ladder resting on floor and against a vertical wall may not be in equilibrium, if

the floor and wall both are smooth surfaces
the floor is rough, the wall is smooth
the floor is smooth, the wall is rough
the floor and wall both are rough sur-faces

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Strength of Materials
If a rigid body is in equilibrium under the action of three forces, then

these forces are equal
the lines of action of these forces meet in a point
the lines of action of these forces meet in a point & the lines of action of these forces are parallel
the lines of action of these forces are parallel

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