[Physics Class Notes] on Equilibrium Pdf for Exam

Equilibrium is generally defined as a state of rest, where there is no change. A body at equilibrium will not experience any positive or negative energy transfers. Biology, physics and chemistry define the state of equilibrium in slightly different terms. But the basic premise is the same. A body at equilibrium will have a minimal influence of external forces on it. Even in the presence of external forces, the opposing forces usually have a balanced effect on the object in concern. Objects at equilibrium are distinguished by their stable nature in terms of motion and energy.

Observing things in their state of equilibrium is not a usual sight since they are acted upon by so many forces both internal and external. Gravity and interatomic forces are the most common examples of forces experienced by objects and due to this, they are usually at unrest if not balanced by an equal and opposite force. This need not mean that the two opposing forces should be equal for a body to be in equilibrium. They just need to cancel each other.

Equilibrium in Physics

The balance of physics is the state of the system when neither the kinetic state  nor the internal energy state of the system changes over time. A simple machine is  in  equilibrium when it receives neither linear  nor angular acceleration. It stays in this particular state indefinitely unless disturbed by  external forces. Equilibrium is established for a single particle when the vector sum of all  forces acting on the particle is zero. By definition, a rigid body that differs from a particle due to its expansion characteristics is called an equilibrium state when the vector sum of all  torques  acting on the object is equal to zero, in addition to the particle state described above. In other words, the state of its rotational motion is constant.

An equilibrium is referred to as stable whenever the small and the externally induced displacements from which the state produces forces tend to oppose the displacement and returns the body or the particle to its state of equilibrium. Examples of this are weights on springs and stones on flat surfaces. Equilibrium is said to be unstable when small deviations produce forces that tend to increase  displacement. An example of this is a ball bearing that is balanced at the edge of the razor blade.  What is physics equilibrium? When equilibrium needs to be defined, the simplest definition is that  the net external force and  torque or other specific point acting on the object around the COM is zero. More specifically, rigid body equilibrium means both  rotational and translational equilibrium. Consider the following equilibrium in the rigid body example. 

Type of Equilibrium 

Equilibrium is classified as dynamic equilibrium or static equilibrium. The next question that arises is  what is static equilibrium and how it differs from dynamic equilibrium. This is known as dynamic equilibrium when the body is in  equilibrium  but continues to move at an unknown speed. For example, a ball that moves at an unknown speed. On the other hand, if the body is in  equilibrium when it is stationary, it is called  static equilibrium.  Equilibrium is not a uniform phenomenon, and there are different types of equilibrium in physics itself. There are three different types of equilibrium. It is also classified  as stable, unstable, and neutral. Let’s take a look at the meaning of all these terms. 

Stable equilibrium is an equilibrium that tends to move towards that particular equilibrium point when the body is out of equilibrium. For example, consider a ball held at the bottom of the hemisphere. In an unstable equilibrium state, when an object moves from that particular point, it moves away from this point. Look at the ball held on top of the ball. As you move, the ball rolls down from the highest point. 

Even in a neutral equilibrium state, the body does not move toward or leave the equilibrium point. For example, move the ball held on the horizontal plane a little. 

Thermal equilibrium in Physics has its distinct definition and properties of equilibrium. As students have seen above, equilibrium in mechanics is defined in terms of net force and net torque acting on a body. But physics encompasses multiple disciplines and thermodynamics is an important field. Thermodynamics deals with the study of heat and related forms of energy and it has its definition of the state of equilibrium. 

According to the branch of thermodynamics, the state of equilibrium is characterized by constant values of macroscopic variables in a system. Said simply, if the important properties like temperature, pressure, volume etc of a system do not change with time, it is said to be balanced and in equilibrium. Equilibrium in thermodynamics is thus visibly distinct from that in mechanics.

Equilibrium can be attained in isothermal, adiabatic processes etc. In each process, equilibrium is defined in different ways depending on the type of energy transfer. Thermal equilibrium is the most common type of equilibrium that students encounter while studying physics. This type of equilibrium is defined in terms of heat exchange and how it is balanced between the system and the surrounding. If there is no heat transfer between the two spaces, the body is said to be in thermal equilibrium. This condition is a very important process in thermodynamics and forms the core condition of many processes specific to physics.

Laws of Thermodynamics 

There are two laws that govern thermodynamics and thermal equilibrium is important to understand the laws. It will be helpful for students if they are familiar with the different types of equilibrium existing within the realm of physics. In this manner, they will be equipped to deal with complex concepts during competitive exams. The zeroth law of thermodynamics deals with three systems that are in thermal equilibrium. The first law of thermodynamics states the condition in which the equilibrium of a system can be disrupted by internal energy transfer and work. A similar understanding of equilibrium can be found in NCERT chemistry textbooks as well if students are looking to form a comprehensive idea of equilibrium that is interdisciplinary. In short, while mechanical equilibrium deals with the idea in terms of net force and net torque that influences the object under concern, thermodynamic equilibrium deals with the energy transfer between the object and its environment. It predominantly focuses on thermal energy (heat) as against mechanical force. Both concepts form the foundation of many concepts, theories and equations in physics.

Students can refer to for queries related to the equilibrium in physics. offers a very comprehensive picture of equilibrium both in general terms as well as in physics. We have paid enough emphasis on the different kinds of equilibrium that is found in the subject. Along with a detailed description of the important concepts relating to equilibrium, students will also find relevant FAQs to aid them in their learning of the chapter. This is important since the concept of equilibrium is a core concept that is at the heart of the discipline of physics and is necessary for pursuing physics for higher studies. ’s material on equilibrium is well suited for this purpose.