Equilibrium

Equilibrium is a condition of a body at rest. When a body is in equilibrium, either there are no external forces acting on it or the sum of all forces and moments equal to zero.

ΣF = 0
ΣM = 0

For example, when placing a book on a table, the book will not fall on the floor but rather stay in equilibrium. Why? In this situation there are two different forces. The book is pressing vertically down on the table and the table is also pushing up on the book. Since both forces are the same in magnitude and in opposite direction, the forces acting on each other cancel out. That is why the sum of forces equal to zero, thus keeping the book in equilibrium.