Static Equilibrium: Definition, Conditions and Examples

Whenever we talk about equilibrium we simply assume that the body is not moving (that is translational equilibrium) but that is not true all the time. Because sometimes the body possesses two types of motion that is translational (motion along a straight line) and rotational (rotating on the axis) as well, for e.g. in case of the wheel of the car. When talking about the complete rest of the system, we come across a term called Static equilibrium. This refers to the translation as well the rotational equilibrium of the system.

Read on to learn about its conditions and its comparison with dynamic equilibrium with some examples.

What is Equilibrium?

In physics, equilibrium refers to a state in which the sums of opposing forces or influences acting on an object are balanced. This means there is no net force or change occurring in the system. Equilibrium can be classified into two main types: static equilibrium and dynamic equilibrium.

Static Equilibrium

Static equilibrium is a state of an object or any system when it remains at rest(that is translational equilibrium) and also neither it rotates or tilts in any direction( that is rotational equilibrium). The word ‘static’ means the body is completely at rest while the word ‘equilibrium’ indicates that all the forces acting on the system are balanced. The forces are considered to be balanced only when the upward directed forces are balanced by downward forces and similarly, the leftward forces are balanced by rightward forces. This doesn’t mean the forces are equal in magnitude.

Importance of Static Equilibrium

Static equilibrium plays a crucial role when it comes to the engineering of rigid structures ranging from the floor system of your house to the giant suspension bridge in the physical world. That is because these structures are expected to maintain the state under all the expected loaded conditions. Engineers need to plan and fabricate structures which don’t fall over, slide, or slant. All in all, they need to ensure their structures stay in static equilibrium: no translational and rotational motions.

Condition for Static Equilibrium

When we say any system or an object is in the state of static equilibrium it must fulfil these two necessary conditions:

• First, the vector summation of all the forces acting on the system must be zero

That is, = ∑ F = 0

It ensures the translational equilibrium of the system. This condition is consistent with both the conditions of static as well as dynamic equilibrium

• Second, the sum of all the external torques acting on the rigid body must be zero. Mathematically, it is represented as,

= 0

This ensures the rotational equilibrium of the system.

Here, τ represents the torque acting on the system. The magnitude of the torque acting on the system can be given as,

= rFsinθ

Where, F = magnitude of the force acting on the system

r = perpendicular distance from the axis of rotation

θ = angle between the force

When both the conditions are satisfied simultaneously by a system or any object we say that the system is in the static equilibrium.

Static Equilibrium Examples

Whenever we talk about static equilibrium, it simply implies the resultant force (net force and net torque) acting on the system is zero and the object is not moving at all in either translational or rotational motion. Here are the examples that we can observe from our surroundings:

• An object, like a book, metal block, etc. lying still on the surface (floor or table surface).
• A weight balancing machine used to be in the state of static equilibrium initially because masses or weight on both sides are equal and hence there is no torque or moment acting.

The same situation happens when there is equal weight on both sides.

• A car parked in the parking lot.
• A man standing still during the National anthem.

Thus, it is clear from the above examples that the objects which are at rest are termed to be in the state of static equilibrium.

Difference between Static  Equilibrium and Dynamic Equilibrium

It is the state of the absolute rest of a body whereas dynamic equilibrium is the state when the net force acting on the system must be zero similar to that of static equilibrium but the system is moving with a constant velocity, that having an unaccelerated motion. Here we have listed some differences between static and dynamic equilibrium:

S.No.	Static Equilibrium	Dynamic Equilibrium
1.	The body is completely motionless.	The motion of the body is unaccelerated, that is it is moving with constant velocity.
2.	The body possesses none of the motions.	The system can move in translatory, rotatory or curvilinear motion.
3.	Both microscopic as well as macroscopic properties remain unchanged	Change in the microscopic state takes place while there is still no change in the macroscopic state.

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