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Stress and Strain

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Topics

  • Introduction
  • Definition: Stress
  • Definition: Strain
  • Formula: Stress
  • Formula: Strain
  • Understanding Elasticity
Maharashtra State Board: Class 11

Introduction

When a force is applied to a body, it causes a deformation (change in shape or size). Inside the body, restoring forces are immediately set up to oppose this change. These internal forces try to bring the body back to its original state. The concepts of stress and strain are used to describe the elastic properties of a body under these conditions.

Maharashtra State Board: Class 11

Definition: Stress

The internal restoring force per unit area of a body is called stress.

Maharashtra State Board: Class 11

Definition: Strain

Strain is defined as the ratio of the change in dimensions of the body to its original dimensions.

Maharashtra State Board: Class 11

Formula: Stress

\[\text{Stress} = \frac{\text{deforming force}}{\text{area}} = \frac{|\vec{F}|}{A}\]
  • \[|\vec{F}|\] is the magnitude of the internal restoring force (which is equal to the external applied deforming force).
  • A is the area over which the force acts.
  • SI unit of stress: N m-2 or pascal (Pa).
  • Dimensions of stress: [L-1M1T-2].
Maharashtra State Board: Class 11

Formula: Strain

\[\text{Strain} = \frac{\text{change in dimensions}}{\text{original dimensions}}\]
Units and Dimensions of strain: Since strain is a ratio of two lengths (or two dimensions), it is a dimensionless quantity and has no unit (based on the provided formula and definition).
Maharashtra State Board: Class 11

Understanding Elasticity

The elastic properties of a body are characterized by its response to an applied force, which leads to stress and strain.

  1. Deformation: When an external force, called the deforming force, is applied to a body, it causes a temporary or permanent change in the body's shape or size.
  2. Restoring Force: As the body starts to deform, internal restoring forces are set up within the material. These forces are directed to oppose the deformation.
  3. Equilibrium: When the body is in equilibrium in its altered shape or size, the magnitude of the deforming force and the internal restoring force are equal and opposite.
  4. Stress: Stress is the measure of these internal restoring forces acting per unit area. It represents the intensity of the forces trying to restore the body to its original state.
  5. Strain: Strain is the measure of the deformation itself. It quantifies how much the body has changed relative to its original size or shape, specifically as a ratio of the change in dimension to the original dimension. This deformation occurs when two equal and opposite forces are applied to an elastic body.

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