**Important Points to remember:**

- Deforming forces are external force applied on a body to change its size or shape.
- Restoring force is the internal force developed in a body trying to bring it back to the original configuration when it is acted upon by an external force trying to change its size or shape.
**Elasticity**is the property of a body by virtue of which it tries to regain its original configuration when the deforming forces are removed.- Perfectly elastic body is a body which regains its original configuration completely on removal of deforming forces.
- Plasticity is the property of a body by virtue of which it
**does not**regain its original configuration on removal of deforming forces. - Perfectly plastic body is a body which does not show any tendency to regain its original configuration on removal of deforming forces
- Stress: The restoring force developed per unit area of a deformed body is called stress.
- Tensile stress: If a deforming force is trying to produce an increase in length, then the restoring force developed per unit area of cross section is called tensile stress
- Compressive Stress: When a deforming force acting normal to a surface tries to produce a change in volume, then the restoring force produced per unit area is called compressive stress.
- Shearing stress: When a tangential force tries to change the shape of a body whose bottom is fixed, then the restoring force developed per unit area is called Shearing stress.
- Strain: The ratio of the change in dimension to the original dimension of a deformed body is called strain
- Linear or longitudinal strain

- is the ratio of change in length to original length.
- Volume strain is the ratio of change in volume to original volume.
- Shearing strain or shear is the angle through which a line originally perpendicular to the tangential force has turned on application of the force.
- Hooke’s Law states that within elastic limit stress is directly proportional to strain.
- Modulus of elasticity is the ratio of stress to strain.
- Young’s Modulus is the ratio of tensile stress to linear strain.
- Bulk Modulus is the ratio of compressive stress to volume strain
- Rigidity modulus is the ratio of shearing stress to shearing strain.
- Lateral strain is the ratio of change in diameter to original diameter when a wire is stretched.
- Poisson’s Ratio is the ratio of lateral strain to longitudinal strain.
- Compressibility is the reciprocal of compressibility.
- Elastic hysteresis is the property of a body by which the body cannot exactly retrace the stress strain graph when the deforming forces are removed.
- Elastic after effect is the delay in regaining the original configuration by a body when the deforming forces are removed.
- Work done in stretching a wire = 1/2 x stretching force x elongation.
- The elastic potential energy per unit volume of a stretched wire = 1/2 x stress x strain

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**One mark Questions**

- Define crystalline solids.
- Define deforming force
- Define restoring force.
- Define elasticity.
- Define plasticity.
- What are perfectly elastic bodies?
- What are perfectly plastic bodies?
- Define elastic limit.
- Define stress.
- Define strain.
- State Hooke’s law.
- Define modulus of elasticity.
- Define Poisson’s ratio.
- Define breaking stress.
- Define ductility.
- Define malleability.
- Define elastic fatigue.
- Define elastic after effect.
- Define elastic hysteresis.
- What are elastomers?

**Two Marks Questions**

- Distinguish ductile and brittle substances.
- Explain why bridges are declared unsafe after long use?

**Three Marks Questions**

- Derive an expression for the elastic potential energy of a stretched wire.
- Describe the origin of elasticity on the basis of the inter-atomic force vs inter-atomic distance curve.
- Show that the elastic potential energy stored per unit volume of a stretched wire is equal to 1/2 x stress x strain.
- Define stress and mention its types. Define each
- Define strain and mention its types. Define each.
- Which are the different moduli of elasticity? Define each and write mathematical expression for each.

**Five Marks Questions**

- Draw the stress vs strain graph of a metallic wire subjected to a gradually increasing stretching force and explain the different regions of the graph.
- Describe the experiment to determine the Young’s modulus of a metallic wire

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