Internal and External Forces: Difference Between Internal and External Forces

By Aina Parasher|Updated : June 1st, 2022

Internal and External Forces: The study of internal and external forces is carried out to understand the concept of free body diagrams which are used to clearly identify the various forces acting on a body in a state of equilibrium. When supports are removed, a body can be subjected to a number of reactions that are exerted on it by the support. These reactions can be identified as either internal or external forces.

In this article, we shall learn that there are various sorts of forces that, when present and involved in doing work on objects, modify the object's total mechanical energy. Other sorts of forces, on the other hand, can never affect an object's total mechanical energy, but can only shift the energy of an object from potential to kinetic (or vice versa). These two categories are called internal or external forces.

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Difference Between Internal and External Forces

The difference between internal and external forces is related to how a system is defined. It's really up to you how you do it, but various approaches are more advantageous in specific situations. Internal force is defined as a force that occurs within a system without any external potency. An external force is a force acting on the system from the environment as a result of external agents. There is one crucial point to remember. Acceleration can only be induced by external forces. For example, you can't take yourself by the hair and lift yourself off the ground because your hand is an extension of your body. As a result, it's impossible to establish a system in which your hand is separate from the rest of your body. If you pull on your hair with your hand, your hair will pull back on the hand. And, because your entire body is connected, there will be no acceleration of the hand-body system's centre of mass in the end. 

Therefore in the case of internal forces, because there is no motion in the system, the system's centre of mass is immovable. Whereas for external forces as the system gains momentum due to external influences, the mass's centre of gravity changes over time. Energy is carried in the form of mechanical energy in internal forces but converted into potential or kinetic energy in the case of external forces. Internal forces working within the system act in opposite directions, canceling each other out and resulting in no net work being done on the system. External forces act in the direction of the imposed force, and work is carried out.

What is an Internal Force?

Internal forces are those forces that hold together the particles of a body. Internal forces reacting within the object do not cause the body to accelerate when it is at rest, but internal actions do result in a change in the system's energy. The potential or kinetic energy of the object is converted to mechanical energy by internal activities, which is conserved by the system. Because the object's acceleration due to internal forces is zero, there is no momentum of the object, hence the work done by the system is always zero, and the mechanical energy is conserved. As a result, the internal force is conservative. The difference between internal and external forces can be understood by an example if we try to pull a bar by applying two equal and opposite forces F (external force) then an internal force S is developed to hold the body together.

What is an External Force?

Newton's First Law of Motion states that "Unless and until some external force is imposed on the body, the object will be in a condition of rest or in a continuous motion at a constant speed." To either accelerate or resist the motion of the body, an external force is required. This could be an applied force, a normal force due to the system's weight, a force related to air resistance, or a frictional force that resists the body's motion and drags it backward. Thus, external forces are the forces that act on a body or a system of bodies from outside. The difference between internal and external forces can be understood by an example.

In order for an automobile to ascend the steeper hill, it must be given more acceleration. More acceleration is required because the internal force of gravity is pointing backward and the frictional force and air resistance draw the automobile down the slope. As the slope increases, the acceleration required to climb also increases. A tension force reacting forward that assists the car in accelerating forward is generated as a result of an increase in the forces acting backwards. Here, tension force, normal force, gravitational force, etc. are the types of internal and external forces acting in this particular example. The extrinsic factors that influence the system, such as the amount of force applied, the normal force due to weight and configuration, the frictional force owing to the surface in touch with the object, the air drag, tensional force, and so on, determine the external forces.

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  • Some examples of internal forces are gravitational force, magnetic force, spring force, electric force, etc. Examples of external forces are friction force, applied force, normal force, tension force, air drag, etc.

  • External forces on a building include wind, earthquakes, the weight of people on the floor of the building, weight of the building. Internal forces on a structure include torque, compression force, turning force, etc. The building is designed such that internal and external forces will not cause it to break or fall.

  • When an earthquake occurs, external factors such as wind, water, and the earth's vibration create internal forces within a structure. Internal and external forces can thus be simply characterized as action and reaction.

  • From the outside, external forces act on buildings. External forces such as strong winds, heavy snow, ice, and the structure's own weight are examples of naturally occurring external forces.

  • A buoyant force is a force that causes an object to float on water. Both internal and external forces act on the object floating on the water surface. The external force of the buoyancy is acting upwards on the body due to the volume of water, but the internal force of the object, gravity, is always acting downward.

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