# When An Equilibrium Is A Static Equilibrium: 9 Facts

When all the components of a system remain at the rest and the net force acting on each particle of that system is zero then the system can be said to be in static equilibrium. Static equilibrium is attained by a system when it is at rest or moving in an uniform motion.

In case of static equilibrium all the forces that are acting on a particular body are used to balance each other. This is the reason why the body does not move and is kept rigid.

If we take an example of a suspended bridge then we will be able to see that all the forces that are acting on the bridge in the upward direction are balanced by the forces that are acting in the downward direction.

## When is a system in static equilibrium?

This is to describe when an equilibrium is a static equilibrium. A body can be said to be at static equilibrium if it is continuing its uniform translational or uniform rotational motion or it is at rest. If we consider the frame of reference of a body then it can be said that if the body neither moves nor rotates then the body is in static equilibrium in that particular frame of reference.

Other than that there are two conditions that are necessary to obtain static equilibrium of a body. They are:

• If the sum of all the external forces that are acting on a body is zero then that body is in static equilibrium.
• If the condition that the net external torque of that body is zeronis maintained then the body is said to be in static equilibrium.

## Why is a system in static equilibrium?

There are a number of reasons when an equilibrium is a static equilibrium. So the first reason is if a body is kept at rest. It means that if a force is acting on that body to keep it in motion then another force is applied to stop its motion. That is why it is at rest.

If the linear or angular accelerations both are zero in case of a body then it is clear that the body is moving in uniform linear motion or uniform circular motion respectively. Other than this it can be at rest. It means the body is in a stationary state.

## How is a system in static equilibrium?

A very common example to describe how a system is in static equilibrium is when a body is kept at rest on a horizontal surface. Actually the main condition of static equilibrium is satisfied here which tells that if two two equal forces are acting on the same body and in an opposite direction then the body is in static equilibrium.

There are two forces that are acting on that body. The first force tries to bring that body into motion whereas the second force is the frictional force acting between the horizontal surface and that body in the opposite direction to resist its motion.

An athelete who is at rest can also be taken as an example of static equilibrium. His rigidity is the result of two forces that are acting on the athelete. So there are two forces that are acting on the body of that athlete. The first force is the non contact gravitational force and the second one is the reaction force which acts in the opposite direction of the gravitational force.

The gravitational force that is the weight of that athlete is acting in the downward direction whereas the reaction force applied by the ground on which the athlete is standing is in the upward direction. These two forces are equal and opposite. Hence the athlete is at rest. ∑F=0 where  ∑F is the sum of these two forces.

## How to know if an equilibrium is in static equilibrium?

There are a number of conditions that should be satisfied by a body,so that it can be termed as a body in static equilibrium. If all these conditions are maintained by a body then it can be said that it is in static equilibrium. The conditions are:

• The sum of all the forces that are acting on the body should be zero. ∑F=0
• The sum of all the external torques that are acting on the body should be zero.  ∑τ=0 where ∑τ is the sum of external torques that are acting on the body.
• The linear momentum of the body should be zero.
• The linear acceleration and angular acceleration should be zero.

These conditions are satisfied by all the larger structures that are buildings,bridges etc. hence they are the examples of a system in static equilibrium.

## When a rigid body is in static equilibrium?

When a rigid body remains in static equilibrium it becomes non deformable in nature. The forces that act on that body are not concurrent in nature. The most important condition that has to be satisfied is that the sum of all the forces must be zero and the the sum of moment of forces should also be zero.

What does it mean if something is in static equilibrium?

There are a number of things that can be concluded from a system which is in static equilibrium. They are:

• The sum of all forces that are acting on the system is zero in every direction.  ∑Fx=0, ∑Fy=0, ∑Fz=0 (in case of a three dimensional system)
• The sum of external torques that are acting on the body is zero in every direction. ∑τx=0 ,∑τy=0 ,∑τz=0
• Linear momentum of the system is zero.
• Linear and angular acceleration of the system is zero.

## Describe a scenario when a system is in static equilibrium

We can take the example of a torque balance. There is a fulcrum which gives support to the horizontal beam within a torque balance. At the two sides of fulcrum masses are to be attached. If the horizontal beam remains rigid or it does not rotate then the system remains in equilibrium. If the beam remains at the level then the system is balanced.

#### Conclusion

This article is entirely based on different facts related to static equilibrium like when a system is in static equilibrium,why a system can be said to be in static equilibrium,how static equilibrium is obtained by a system,examples of static equilibrium etc.

Ankita Biswas

Hi...I am Ankita Biswas. I have done my B.Sc in physics Honours and my M.Sc in Electronics. Currently, I am working as a Physics teacher in a Higher Secondary School. I am very enthusiastic about the high-energy physics field. I love to write complicated physics concepts in understandable and simple words. Let's connect through LinkedIn:https://www.linkedin.com/in/ankita-biswas-b95785230