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# HC Verma for Class 11 Physics Chapter 14:Some Mechanical properties of matter

Chapter 14 Some Mechanical properties of matter deals with the entities of matter. Mechanical properties of matter include stress, strain and Young's modulus. Stress can be defined as the internal restoring force acting per unit area of a deformed body. Thus, Stress = Restoring force / Area. The forces between different atoms are responsible for the structure of a molecule, and the forces between the molecules are responsible for the structure of the material. The property that restores the natural shape or opposes the deformation is called elasticity.

Stress is of two types, normal and tangential stress. When the deforming force is applied normal to the area, then the stress is called normal stress. When the deforming force is applied tangentially, it is called tangential stress. The strain takes place when there is a fractional change in configuration. Hooke's law is also discussed in this chapter. If the deformation is small, the stress in a body is proportional to the corresponding strain. It can be expressed as Stress =E* Strain where E is known as the modulus of elasticity of the material of the body. Modulus of elasticity is classified into three types.

• 1. Young's Modulus of Elasticity is the ratio obtained between normal stress and longitudinal strain within the elastic limit. It is expressed as Y = Normal stress / Longitudinal strain.
• 2. Bulk Modulus of Elasticity is the ratio obtained between normal stress and volumetric strain within the elastic limit.
• 3. Modulus of Rigidity is the ratio between tangential stress and shearing strain within the elastic rigidity. It is expressed as η = Tangential stress / Shearing strain

All objects have their limit of elasticity. The angle formed between the tangent planes at the solid surface and the liquid surface at the contact is called the contact angle. For the liquid that rises along the solid surface, the contact angle is smaller than 90°. Conversely, for the liquid that is depressed along the solid surface, the contact angle is greater than 90°. A brief description of Viscosity and Stoke's law has been included.