Principle of equilibrium:

Sol.: Principle of equilibrium can be divided in to three main parts;

(1) Two Force Principle: As the Resultant is zero when body is in equilibrium, so if the two forces are acting on the body, then they should be equal, collinear and opposite.

(2) Three Force Principle: According to this principle, if a body which is in equilibrium is acted upon by three forces, and then the resultant of any two forces should be equal, opposite and collinear with the third force. For finding the values of forces usually we apply lamis theorem

(3) Four Force Principle: According to this principle, if four forces act upon a body which is in equilibrium, then the resultant of any two forces should be equal, opposite and collinear with the resultant of the other two.

And for finding out the forces we apply;

∑H = ∑V = 0, because resultant is zero.