We may say that an object at rest is in equilibrium or in static equilibrium. An object at rest is described by Newton's First Law of Motion. An object in static equilibrium has zero net force acting upon it.
The First Condition of Equilibrium is that the vector sum of all the forces acting on a body vanishes. This can be written as
F = F1+ F2+ F3+ F4+. . . = 0
where , the Greek letter sigma, again means the summation of whatever follows -- the summation of the forces, in this case.
An object in equilibrium does not move along a straight line -- it does not translate -- that means the sum of all the forces on it is zero. That was the first condition of equilibrium.
But an object in equilibrium also does not rotate. That means the sum of all the rotational forces on it is also zero. The sum of all the torques on an object is equilibrium is zero. This is the Second Condition of Equilibrium.
1st law of motion.... Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
2nd law of motion...
The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.
3rd law of motion..... For every action there is an equal and opposite reaction.