Terms in Physics: Stress and Strain

When a body is subjected to a deforming force, a restoring force occurs in the body which is equal in magnitude but opposite in direction to the applied force. This restoring force per unit area is known as stress. We can also refer to stress as a measure of the internal force experienced by an object per unit of cross-sectional area.

If $F$ is the force applied and $A$ is the area of cross-section of the body, then

$$\text{Magnitude of stress}=\dfrac{F}{A}=\dfrac{\text{Force}}{\text{Area}}.$$

SI unit of stress is $N \ m^{-2}$ or pascal $(Pa)$ and its dimensional formula is $[ML^{-1}T^{-2}]$ .

Strain is simply the measure of how much an object is stretched or deformed. Strain occurs when force is applied to an object. Strain deals mostly with the change in length of the object.

If the original length of the body $L_0$ changes by $\Delta L$ , then stress can be expressed as

$$\text{Strain}=\dfrac{\Delta L}{L}=\dfrac{\text{Change in Length}}{\text{Original Length}}.$$

Since strain is the ratio of two quantities with the same dimensions, it has no unit.

Types of stress:

• Longitudinal stress, which consists of tensile stress and compressive stress
• Tangential or shearing stress
• Hydraulic stress

Types of strain:

• Longitudinal strain
• Shearing strain
• Volume strain

When a body is stretched by two equal forces applied normal to its cross-sectional area, this restoring force per unit area is called tensile stress.

When a body is compressed under the action of applied forces, this restoring force per unit area is known as compressive stress.

Tensile or compressive stress is also known as longitudinal stress.