When An Equilibrium Is A Dynamic Equilibrium: 9 Facts

We know that equilibrium means balancing the system. There are mainly two types of equilibrium called dynamic and static equilibrium.

The dynamic equilibrium describes the stability of a system under the balanced condition of all the forces. Several conditions should be satisfied to describe an equilibrium as a dynamic equilibrium. In this post, we will learn when an equilibrium is a dynamic equilibrium and several facts involved in dynamic equilibrium.

In any physical system, there must be some forces exerted on them. These exerted forces correlated to the equilibrium. Depending on the behavior of forces, the dynamic equilibrium can be derived.

File:Bicycle and motorcycle dynamics Stability 2C.png - Wikimedia Commons when an equilibrium is a dynamic equilibrium
Image of when equilibrium an is a dynamic equilibrium
Image credits: Wikimedia commons

When a system is in dynamic equilibrium?

A system is said to be in dynamic equilibrium when the system moves with constant velocity and acceleration is zero such that there is no net force acting on the system. Generally, the object under uniform motion is said to have dynamic equilibrium.

The term dynamic itself means motion. If an object is under uniform motion, the velocity remains constant so that there is no acceleration is possible. Under such conditions, according to Newton’sNewton’s second law of motion, the net force acting on the object becomes zero, causing a dynamic equilibrium condition.

In other words, dynamic equilibrium is also referred to as steady-state equilibrium. When there is no observable change occurring in the system, then the dynamic equilibrium is established in the system.

In thermodynamics, dynamic equilibrium is considered the time average of random motion of the small object under constant velocity.

Why a system is in dynamic equilibrium?

Since we know that equilibrium means balance, the system must remain in the same state under equilibrium conditions. The dynamic equilibrium is exerted on the object under motion by balancing forces in all directions. In order to retain the motion of the object, the system will be in dynamic equilibrium.

The equilibrium can also be described as the no-change state in the physical system. If the object’s velocity keeps changing, the object begins to accelerate, and thus net force varies with the acceleration. To maintain the constant motion of the object, the system will be in dynamic equilibrium.

How a system is in dynamic equilibrium?

When two equal and opposite forces are exerted on the moving object, the velocity of the object becomes constant. Thus net force will be zero by canceling the exerted force. This makes the system attain dynamic equilibrium.

Any physical system with no net force under constant velocity is said to be in dynamic equilibrium –this means that the system is under translational motion. Under such conditions, the dynamic equilibrium is established in the system if there is a uniform speed.

In another context, when the rate of the forward reaction is equal to the backward reaction, the dynamic equilibrium occurs.

[latex]A+B\rightleftharpoons AB[/latex]

Where does dynamic equilibrium happen?

The dynamic equilibrium happens when the body is under rectilinear motion with constant velocity. Even if a body is under uniform circular motion, the angular velocity of the body will be constant, and we can observe dynamic equilibrium in that case.

You can observe this in trains under constant velocity where net force acting on the train remains unaltered and a uniformly rotating fan with constant speed. It does not accelerate, so the angular velocity of the fan is constant, and the net force is zero.

If we consider thermodynamics, the dynamic equilibrium generally occurs in a closed system. The dynamic equilibrium can also be stated as “the condition in which if any reaction occurs, the original composition does not change with time.”

The above statement means that the transition between the reaction takes place, the forward reaction and backward reaction must be equal. So in a reversible reaction, we can get dynamic equilibrium.

How to know if an equilibrium is in dynamic equilibrium?

If any object is not accelerating, there are more chances for the object to attain dynamic equilibrium because we know that constant velocity always provides dynamic equilibrium conditions for the system.

The forces acting on the system are equal and opposite in dynamic equilibrium. Suppose force Fa is acting on the body under motion means some force Fb is acting opposite to Fa to balance the system so that velocity can be set constant.



For example, the treadmill we used in the gym. The person has the constant velocity as his legs move in the forward direction, and the treadmill ramp is moving backward. The force exerted by the leg to cause forward motion is equal to the force exerted on the treadmill to cause backward motion. This action causes the person to run with constant velocity with dynamic equilibrium.

Treadmill maintaining dynamic equilibrium
Image credits: Wikimedia commons

What happens when a mixture is at dynamic equilibrium?

The dynamic equilibrium occurs only when the reaction is reversible; that means even after the reaction takes place and you reverse it, you will end up with the original components with the same initial concentration.

In dynamic equilibrium, the mixture remains the same with the same quantity and concentration of both reactant and product. Such that the mixture reaches the dynamic equilibrium, you cannot add or remove any substance to the mixture. Only energy can be involved in the process at dynamic equilibrium in the mixture.

For example, consider a sugar-water mixture. The sugar-water when they are in equilibrium, the sugar keeps on dissolving in water with no change in the initial quantity of the sugar. Even if you re-crystallize the sample, you will initially obtain the same amount of sugar you have added.

When a liquid is in dynamic equilibrium with its vapor?

When a liquid is in dynamic equilibrium with its vapor, the rate of condensation and evaporation are equal to each other such that the two phases share the same amount of kinetic energy at the same temperature. We know that vapor is the gaseous state of the liquid. Since it is just a phase transition of the liquid, the dynamic equilibrium always correlates with the liquid and vapor.

Assume you are keeping a liquid in a closed container with zero vapor pressure, and the rate of condensation is also zero, but the rate of evaporation is not at all zero. But as the time changes, the vapor pressure rises until the condensation rate becomes equal to the evaporation rate keeping the system at a dynamic equilibrium condition.

Evaporation and condensation_distilling | Illustration used … | Flickr
When liquid is in dynamic equilibrium with vapor
Image credits: Flickr

Describe a scenario when a system is in dynamic equilibrium

The system in dynamic equilibrium can be explained using a skydiver as an example. When a skydiver jumps, gravity pulls him towards the center of the earth so that he begins to accelerate. But as the speed increases, the air resistance or drag which has the same magnitude but is exerted opposite to the gravity sets the dynamic equilibrium condition.

The drag exerted in the opposite direction eventually becomes equal to gravitational force such that the net force on the diver keeps on decreasing until it reaches zero along with the acceleration.

Even though acceleration becomes zero, the diver keeps moving because of increasing the velocity. The velocity is called terminal velocity, which makes the diver fall as fast as he can.

File:Terminal velocity.svg - Wikimedia Commons, when equilibrium is dynamic equilibrium
Free body diagram of dynamic equilibrium
Image credits: Wikimedia commons


In this post, we learned that dynamic equilibrium is associated with zero net force and constant velocity. The reaction rate must remain constant even when the reaction is reversed.

Keerthi Murthi

I am Keerthi K Murthy, I have completed post graduation in Physics, with the specialization in the field of solid state physics. I have always consider physics as a fundamental subject which is connected to our daily life. Being a science student I enjoy exploring new things in physics. As a writer my goal is to reach the readers with the simplified manner through my articles. Reach me – keerthikmurthy24@gmail.com

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