Note on Terms related to elasticity

  • Note
  • Things to remember

Deforming force
The external force which changes the shape and size of the body is defined as the deforming force. Or, the force which produces deformation in the body is known as deforming force. For example: When a rubber is stretched with an external force, the force acting on the rubber is deforming force and the stretched state of the rubber is the deformed state.

Elastic body: The body which can regain its original shape and size after the removal of deforming force is known as deforming force is known as elastic body. For example: rubber, string, etc.

Perfectly elastic body: The body which can regain its original shape completely and instantaneously after the removal of deforming force is called the perfectly elastic body. There is nobody in the universe which is perfectly elastic in nature. Perfectly elastic body is just an ideal concept. For example: quartz fiber is the nearest approach to perfectly elastic body.

Elasticity: The property of a body by virtue of which it tends to regain its original shape and size is known as elasticity.

Plastic body: The body which cannot regain its original shape and size after the removal of deforming force is known as plastic body.

Perfectly plastic body: The body which cannot regain its original shape and size after the removal of deforming force, however small the deforming force is applied is called perfectly plastic body. There are no bodies in the nature which is perfectly plastic. For example: paraffin wax, clay, etc. are nearest approach to perfectly plastic body.

Plasticity: Plasticity is the property of a body by virtue of which it does not tend to regain its original shape and size after the removal of deforming force.

Restoring force: The force which is responsible to regain the original shape and size of the body is called restoring force. For example: when a rubber is stretched by a deforming force, a force acts in opposite direction within the rubber which tries to bring the rubber in its original position. This force is called restoring force.

Stress: Restoring force developed per unit area of the body is known as stress. Since restoring force is equal and opposite of the deforming force, we may define stress as deforming force per unit area of the body.

$$\text {i.e.} \text {Stress} =\frac {\text {restoring force or deforming force}}{\text {area}} $$

The SI unit of stress is Nm-2

Dimensional formula = [ML-1T-2]

Depending upon how we have applied force, there are different types of stress. They are:

  1. Normal stress
    When force is applied perpendicular to the surface of the body, the applied force per unit area of the body is defined as normal stress. There are two types of normal stress
    Tensile stress in a rod
    Tensile stress in a rod


  2. Tensile stress
    If the force applied on the body increases the length of the body or produces extension in the body in the direction of force applied, the stress set up in the body is called tensile stress. The attractive intermolecular forces give rise to tensile stress.

  3. Compression stress
    If the force applied on the body decreases the length of the body, then the stress set up in the body is called compression stress. The repulsive intermolecular forces give rise to compression stress.

  4. Tangential stress or shearing stress
    When two equal and opposite forceareactingtangential to the two opposite faces of the body as showninfigure above, one face is displaced with respect to the other face, the stress set up in the body is called tangential stress.Therefore, theratiooftangential force acting on the surface to the area of the surface is called tangential stress. This type of stress changes the shape of the body.
    Tangential stress.
    Tangential stress.


  5. Volume stress or bulk stress or hydraulic stress
    When a body is immersed in a fluid, the fluid exerts force from all directions. Due to this, the volume of the body decreases and the stress set up in the body is known as volume stress.
    (a) Body outside the fluid (b) Bulk stress
    (a) Body outside the fluid (b) Bulk stress

Therefore, volume stress is defined as the force acting perpendicular to the surface of the body per unit area of the body.

  • The dimensional formula of stress is [ML-1 T-2]  
  • The property of a body by virtue of which it tends to regain its original shape and size is known as elasticity.
  • The body which can regain its original shape and size after the removal of deforming force is known as deforming force is known as elastic body.
  • When a body is immersed in a fluid, the fluid exerts force from all directions.
  •  Restoring force developed per unit area of the body is known as stress .

 

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