Equilibrium can be stable or unstable depending upon how long the system can remain in its state of equilibrium. A body in equilibrium that is disturbed, but comes back to equilibrium is said to be stable. Consider stirring tea in a mug. It becomes still after sometime. A body in equilibrium that is disturbed, and loses it state of equilibrium permanently is said to unstable. Consider a tennis ball balanced on a racket. It can fall if moved a little. When an object is disturbed by external forces which impart acceleration to it, it loses its stable state of equilibrium. This article will help you with static equilibrium calculations. Equal and Opposite Forces A body under the influence of equal and opposite forces is not always in equilibrium. Consider the following two cases. If all the forces acting on a motionless object are balanced, it is said to be in static equilibrium. To understand the concept of static equilibrium is it important to determine the forces acting on the object and, their directions. On determining the forces, we must find the vector sum of these forces. Examples of Balanced Forces Book placed on a table A book placed on the table experiences two forces, the upward force of the table and downward force of gravity. These forces are in opposite directions and have the same magnitude thus, balancing each other. Hence, the book is considered to be in a state of equilibrium. Weight Hung by multiple strings In the diagram given below, an object is hung by three strings. To understand its state we need to calculate the forces acting upon it. If the net sum of all the forces acting on the object is zero, it implies that the object is in equilibrium. Forces act on individual points of an object. However, this fact makes the summation of forces difficult. Hence for simplifying the whole concept of equilibrium, the object's mass is considered to be one single number rather than discrete points. Problem I: Consider a point upon which three forces with magnitudes 4N, 8.2N, and 9.5N act at angles 150^o, 65^o, 270^o respectively. Calculate the net force. Initially, try to draw a composite diagram depicting all the forces involved. For the above problem the resultant composite diagram of the forces is as follows: Finding the components of force along the X and Y axis. For a rigid body to be considered in the state of static equilibrium, following two conditions need to be followed. CASE I If the vector sum of all the forces acting on the object is zero, implies that object is in translational equilibrium. In other words net external force should be zero. S(F) = 0 CASE II If the sum of torques acting externally on the object is zero, implies that the object is in rotational equilibrium. In other words net external torque should be zero. S(t) = 0 When both the above conditions are satisfied implies that the rigid body is in static equilibrium. Problem II: Consider the system in the given figure where in two masses are suspended by strings. This system is in the state of static equilibrium. Calculate Ta, Tb, Tc, and the angle Θ. Solution: Remember that the sum of forces and, torque is always equal to zero. Consider the object suspended on the left side of the system. Consider the object suspended on the right side of the system. A photo frame hung on a wall, a glass kept on the table, or a bridge, do not undergo translational or linear motion at any point. Hence, we can say that they all follow the concept of static equilibrium.