Internal Forces: Types, Factors, Examples of Internal Forces

By Aina Parasher|Updated : August 3rd, 2022

Internal Forces such as compression and tension play a very important part in our day-to-day life. Forces can be broadly classified into two categories: internal and external forces. When specific types of forces are present and involved in working on objects, the overall mechanical energy of the object is altered. On the other hand, some sort of force 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 are termed Internal forces and external forces. The Internal Forces can change KE to PE. The total work done by internal forces is always zero.

When external forces are operating on a body, a system of internal forces and internal moments develops within the body to counteract the external forces. Internal forces and moments must be understood to comprehend how a body will deform and if it will break under stress. In this article, we will study in detail internal forces, factors on which internal forces are dependent, and various examples that we experience in our day-to-day life.

Table of Content

What are Internal Forces?

Internal forces are the forces that the bodies in the system exchange. Internal forces may generate acceleration in various system areas but may not cause acceleration in the system's center of mass.

Internal Forces Meaning

Internal forces can change a system's energy. Internal forces are the forces that hold a body's particles together. The total work done by internal forces does not affect the work done by the system as it always remains zero.

Internal forces reacting within the object do not cause the body to accelerate when it is at rest, but internal forces 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. 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.

Types of Internal Forces

A force that acts from within a body is known as an internal force. The internal force can be divided into 4 types depending upon the reaction produced within the body in a system:

  1. Compression: Compression is a squeezing force applied to a material. This strain frequently shortens materials. For example, in your hand, squeeze a sponge. You're generating compression.
  2. Tension: Tension is a force that causes a substance to stretch. This strain frequently lengthens materials. For example, hold one end of an elastic band in each hand. Start distancing your hands. You're stretching the elastic band by creating tension.
  3. Torsion: Torsion is a force that twists or turns. For example, the twisting force of torsion is used to wring out a damp washcloth. To force out the water, you grab the washcloth in two hands and twist it in different directions.
  4. Bending: Bending is the process of bending a straight material into a curved shape. One side of the material is pressed against the other (compression). The other side spreads out (tension).
  5. Shear: Two nearby forces pushing or pulling each other, but not directly opposing each other. By moving an object's molecules apart sideways, a shearing stress slice or rips it (for example, pruning shears cutting through a branch, paper-cutter cutting paper - the branch and paper are "subjected to a shear loading").

Internal Forces Example 

When the wind blows on a tree, it swings back and forth. Because it comes from the outside, the wind's force on the tree is considered an external force. Internal force, on the other hand, is what keeps the tree in place and keeps it from falling down. Another example is when the margins of a scale bend when a muscular force is applied to them. There is a considerable deal of stress and compression on this scale. The muscle force acting on the scale is the external force. The magnitude of this force is large enough to bend but not destroy the scale. This is because it is held together by an internal force that prevents it from collapsing.

When a bus is pushed from the inside while passengers are inside, it does not move. On the other hand, the bus tends to move when passengers exit and apply push force from the outside. An external force the passengers apply causes motion to be introduced into the bus. The passengers' push force while seated inside the bus does not assist the vehicle in moving because the passengers have become a part of the system. An internal force is a force that keeps a system from moving.

When a force is applied to the spring to stretch it, the spring moves. The external force is the force that results in the movement of the spring. The internal force is the force that tends to compress the spring and restore its original shape. The internal force opposes mobility and any change in shape, and it is diametrically opposed to the outside force.

Factors on Which Internal Forces Depend

The amount of force imposed on a body and the intrinsic and extrinsic variables are the factors on which both internal forces and external forces are dependent. It is essential to know that net work done by internal forces is frame invariant, which means it won't change even if the frame of reference is changed.

The dipole moments, internal heat of the system, emissivity, temperature of the system and its surroundings, composition, weight, density, the separation between the molecules constituting the system, motion of the particles in the system, geometry of the system, molecular constituency, covalent bonds between the atoms, number of free particles, and other factors all influence the internal forces within the system.

How Does Internal Force Act on a System?

Internal forces in the system work oppositely, cancelling each other out and resulting in zero output. Internal forces appear to primarily oppose changes brought on by external forces or in response to external agents such as electric, magnetic field interaction, or temperature change.

When an electric field is applied to a conductor, the charged particle moves in a helix but does not create any external changes to the item or cause the centre of mass to accelerate. Due to the spin of the electric particle, the magnetic field is produced by the motion of the charged particle. When a material with magnetic properties is placed in a magnetic field, the dipoles are arranged in the field's direction. Internal magnetic flux lines cause the magnetic spin dipoles to align following the field.

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FAQs on Internal Forces

  • Internal forces are the forces experienced or applied by the body inside a system. Internal forces do not change the acceleration of the system. Internal forces are conservative in nature.

  • In order to calculate internal forces, first draw a free body diagram of the entire system, find the support reactions (external forces), and keep all the loads in their original locations. At the point where the internal loads are to be determined, cut a section through the member perpendicular to its axis. A free-body diagram of the cut section is then drawn to determine the internal forces using equations of static equilibrium.

  • In 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.

  • The 5 important types of internal forces are-

    • Shear
    • Bending
    • Torsion
    • Compression
    • Tension.
  • If internal forces in a system are not balanced they will cause deformation of the body. In mechanics, we consider non-deformable rigid bodies. Internal forces are therefore always considered to be balanced.

  • Endogenic forces are forces that operate within the earth's surface. Pressures that exist within the earth due to endogenic forces are also known as Internal forces. Such internal forces cause subsidence, land upliftment, volcanism, faulting, folding, earthquakes, and other vertical and horizontal motions.

  • The total work done by internal forces is always zero. For rigid bodies, it remains zero.

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