Equilibrium Physics

Equilibrium in physics refers to the condition of the system when neither of its state of motion nor its internal energy state changes with the time. A simple mechanical body is known to be in the state of equilibrium when it experiences neither the linear acceleration nor the angular acceleration. Unless it is disturbed by an external force, it will continue in that particular condition indefinitely. For one single particle, the equilibrium arises when the vector sum of all the forces acting upon the particle results as zero. A rigid body, which is by definition differentiated from the particle in having the property of the extension, is said to be in equilibrium when, in addition to the states listed for the particle above, the vector sum of all the torques that are acting on the body equals to zero so its state of the rotational motion is constant. An equilibrium is referred to as stable whenever the small and the externally induced displacements from which the state produce forces tend to oppose the displacement and returns the body or the particle to its state of equilibrium. Examples of this include the weight that is suspended by a spring or the brick lying on a levelled surface. An equilibrium is said to be unstable when the least departure produces the forces which tend to increase the displacement. An example of this is a ball bearing that is balanced on the edge of the razor blade.

What is Equilibrium in Physics

Let us learn about the equilibrium definition physics.

When you have to define the equilibrium the most simple definition is that it is a point wherein the net external force and the torque acting on the body about COM or any other given point results to zero. But to be more specific for the rigid body equilibrium means both the rotational as well as the translational equilibrium. Consider the following equilibrium of a rigid body examples:

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Types of Equilibrium

Equilibrium is classified as the dynamic or the static equilibrium. The next question that arises is that what is static equilibrium and how is it different from the dynamic? Hence, when the body is in the equilibrium state but still continues to move with an unknown velocity it is called as the dynamic equilibrium. For example, a ball that moves with an unknown velocity. On the other hand, when the body is in the equilibrium state while being at rest it is called as the static equilibrium.

Equilibrium is classified also as stable, unstable and neutral. Let us see what all these terms mean. 

A stable equilibrium is the one in which when the body is displaced from its equilibrium state it tends to move towards that specific equilibrium point. For example, consider a ball that is kept at the bottom of a hemisphere. While in the case of the unstable equilibrium, when it is displaced from that specific point the body moves away from the point. Consider a ball that is kept at the top of the sphere. When you slide it, the ball rolls away from the topmost point.  Similarly, in the case of the neutral equilibrium, the body neither moves towards nor moves away from the equilibrium point. For example, displace a ball that is kept on the horizontal surface slightly.

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FAQ (Frequently Asked Questions)

1. What Is The Equilibrium Of A Body?

A system is said to be in the stable state of equilibrium when it is displaced from equilibrium, it experiences the net force or the torque in such a direction that is opposite to the direction of the displacement. For example, consider a marble at the bottom of a bowl. It will experience a restoring force when it is displaced from its original equilibrium position. This force tends to move it back towards its equilibrium position. Most of the systems are in stable equilibrium, especially in the case of the small displacements.

2. What Is The Equation For Equilibrium?

The equation of the equilibrium states that the net force must be zero

The net force that is acting on the object should be zero. Hence, all the forces balance in each of the direction. For example, consider a car that is moving along the highway at a constant speed. It is said to be in the state of equilibrium, as it does not accelerate in any forward or any vertical direction. Mathematically, this equation is stated as follows:

Fnet = ma = 0

3. What Are The 3 Types Of Equilibrium?

Stability refers to the state of the rest of the body while the equilibrium is known as the state of the balance of a body. When a specific body is in the state of the equilibrium, it stays in that particular position for as long as there is no external force or influence on it.

There are three different states of equilibrium which are as follows:

  1. Stable equilibrium

  2. Unstable equilibrium

  3. A neutral equilibrium

4. Which Is Always True For A Body In Equilibrium?

The conditions for equilibrium of a rigid body state that for an object to be in the state of the equilibrium, it should be experiencing no amount of the acceleration. This means that both the net force and the net torque that is acting on the object should be zero. This condition is always true for a body which is in the state of equilibrium. It is given by the expression Fnet = ma = 0.

For achieving this condition, the forces which are acting along each of the axes of the motion should sum up to zero. For example, the net external forces that are acting along the typical x– and y-axes are zero. This is written in the form of an equation which is given as follows:

The net Fx = 0 and the net Fy = 0.