Internal and External Forces: Difference Between Internal and External Forces

By BYJU'S Exam Prep

Updated on: September 25th, 2023

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. Internal and external forces come into action when internal or external agents apply them. These agents are the sole decision-makers in the consideration of a force as an internal or external force.

Various forces that, when present and involved in doing work on objects, modify the object’s total mechanical energy. On the other hand, other sorts of forces can never affect an object’s total mechanical energy. Still, they can only shift the energy of an object from potential to kinetic (or vice versa). These two categories are called internal and external forces. This article will specify the differences between internal and external forces in detail.

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Internal and External Forces

Internal and External forces depend on whether the object applying force is inside or outside the system. Forces applied from internal objects are known as an internal force, whereas forces applied by external sources are known as external forces.

If we want to specify two differences between internal and external forces, then the first would be that internal forces can not change the acceleration of the system’s center of mass, whereas external forces can. The other difference between internal and external forces would be that total work done by internal forces is frame invariant, whereas work done by external forces depends on the frame of reference.

Difference Between Internal and External Forces

The difference between internal and external forces relates 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 due to 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 center of mass in the end.

Therefore, in the case of internal forces, the system’s center of mass is immovable because there is no motion in the system. Whereas for external forces, the mass’s center of gravity changes over time as the system gains momentum due to external influences. 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 at rest, but internal actions change the system’s energy.

The potential or kinetic energy of the object is converted to mechanical energy by internal activities, which are 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, 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. An external force is required to accelerate or resist the body’s motion. 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.

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 due to an increase in the forces acting backward. Here, tension, normal, gravitational, 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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