# CBSE Class 11 Physics Chapter 9 Revision Notes

Chapter 9: Mechanical Properties Of Solids Revision Notes

**Inter molecular Forces:**In a solid, atoms and molecules are arranged in such a way that the forces exerted by neighboring molecules act on each molecule.**Inter molecular forces**are the name for these forces.**Elasticity:**Elasticity is the ability of a body to return to its original configuration (length, volume, or shape) when deforming forces are removed.- The forces between a body’s atoms or molecules determine how it changes shape or size when external forces act on it.
**Elastic forces**are atomic forces that have a short range.**Perfectly Elastic Body**: A perfectly elastic body is one that returns to its original configuration immediately and completely after a**deforming force**is removed from it. Quartz and phosphor bronze are two examples of nearly elastic materials.**Plasticity**is the inability of a body to return to its original size and shape even after the deforming force has been removed, and such a body is referred to as a**plastic body**.**Stress:**The ratio of the internal force F produced when the substance is deformed to the area A over which this force acts is defined as**stress**. This force is equal in magnitude to the externally applied force when it is in equilibrium.

- There are
**two types of stress**:

**(i) Normal stress**: The restoring force per unit area perpendicular to the body’s surface is defined as normal stress. Tensile and compressive stress are the two types of normal stress.

**(ii) Tangential stress**: Tangential stress occurs when the elastic restoring force or deforming force acts parallel to the surface area.

**Strain:** The ratio of the change in size or shape to the original size or shape is what it’s called. It is just a number with no dimensions.

- There are three types of strains:

**(i) Longitudinal strain**: The strain produced in the body is called longitudinal strain or tensile strain if the deforming force produces only a change in length. It is written as follows:

**(ii) Volumetric strain**: When a deforming force causes only a change in volume, the strain in the body is referred to as volumetric strain.

**(iii) Shear strain**: Shear strain is the angle tilt in the body caused by tangential stress expressed. It is written as follows:

**Hooke’s Law**: Hooke’s law states that within elastic limits, the ratio of stress to the corresponding strain produced**is constant**. The modulus of elasticity is the name given to this constant.

**Stress Strain Curve**: Stress-strain curves are useful for determining a material’s**tensile strength**. A stress-strain curve for a given metal is depicted in the diagram.

**Young’s Modulus**: Young’s modulus of elasticity within the elastic limit is defined as the**ratio of**longitudinal stress to longitudinal strain for a solid in the form of a wire or a thin rod. It is written as follows:

**Bulk Modulus**: The bulk modulus is defined as the**ratio of**longitudinal stress to volumetric strain within the elastic limit. It is written as follows:

- The term
**“compressibility”**refers to the reciprocal of bulk modulus. The fractional change in volume per unit change in pressure is what it’s called. **Rigidity Modulus or Shear Modulus:**It is defined as the tangential stress divided by the shear strain.

**Poisson’s Ratio**:**Lateral strain**is defined as the ratio of the change in diameter (D) to the original diameter (D).**Longitudinal strain**is defined as the ratio of the change in length (l) to the original length (l).**Poisson’s ratio**is the ratio of lateral strain to longitudinal strain.