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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

One mark Questions

1. Define crystalline solids.
2. Define deforming force
3. Define restoring force.
4. Define elasticity.
5. Define plasticity.
6. What are perfectly elastic bodies?
7. What are perfectly plastic bodies?
8. Define elastic limit.
9. Define stress.
10. Define strain.
11. State Hooke’s law.
12. Define modulus of elasticity.
13. Define Poisson’s ratio.
14. Define breaking stress.
15. Define ductility.
16. Define malleability.
17. Define  elastic fatigue.
18. Define elastic after effect.
19. Define elastic hysteresis.
20. What are elastomers?

Two Marks Questions

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

Three Marks Questions

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

Five Marks Questions

1. 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.
2. Describe the experiment to determine the Young’s modulus of a metallic wire