The idea of a hard body, where the constituent molecules are fixed with respect to each other, is theoretical. Till how we were operating with a hard body because most solids have very little deformation on function of relatively small forces. But if the forces are big enough we cannot ignore the deformation.
(i) Deforming Force: If a force applied on a body causes a modification in the normal positions of the molecules of the body, resulting in a modification in the configuration of the object either in length, shape or volume, then the force applied is deforming in behavior.
(ii) Elasticity: The characteristic of a body by virtue of which the body takes its real configuration when external deforming forces are removed is known as elasticity.
(iii) Cause of Elasticity: Particles in a body are bound to one another by forces which is electrostatic in behavior. When elongated the bonds seem like springs because of inter molecular interaction. When subjected to compression beyond a specific distance or less than a specific inter molecular separation, the molecules, instead of attaching each other, repel because of inter nuclear forces.
(iv) Interatomic force constant (K): Interatomic force constant is described as ratio of interatomic force to modification in interatomic distance. K = Young's modulus * interatomic distance
(v) Perfectly Elastic Body: A body which gets its actual configuration immediately and completely after the removal of deforming force from it, is known as perfectly elastic body. Phosphor bronze and quartz are the types of nearly perfectly elastic materials. Liquid glasses an kind of perfectly plastic object.
(vi) Perfectly Plastic Body: An object which does not get its real configuration at all on the removal of deforming force, however short the deforming force can be is a perfectly plastic body.
(vii) STRESS: Stress is calculated by the deforming force per unit normal area.
Stresses set up in an object because of the action of forces can be divided into three broad categories
(a) Tensile stress, which gives elongation, is the answer of forces operating on the body along a single direction.
(b) Shear stress, which gives "shear or bending" being the result of same and opposite torque about the origin of mass operating on a body.
(c) Volume stress (Bulk Stress) which produces a change in the volume of the body - the resulting strain is known as volume strain (Bulk Strain).
(d) Unit of stress = F/A= N/m2
(e) Dimension of stress = ML-1T-2
(viii) STRAIN: When deforming forces are applied to a body, there is a modification in the size of the body. The object is called strained or deformed. The ratio of modification in dimension to the original dimension is called strain. i.e.
Strain =Change in dimension/Original dimenstion
Strain being the ratio of two similar values, so it has no units and size. There are three kinds of strain
(a) Longitudinal strain = Change in length/Original length =ΔL/L
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![809_Elasticity.png](https://www.expertsmind.com/../CMSImages/809_Elasticity.png)
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(b) Volume strain =Change in volume/Original volume =ΔV/V =
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![2412_Elasticity1.png](https://www.expertsmind.com/CMSImages/2412_Elasticity1.png)
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(ix) ELASTIC LIMIT: Elastic limit is the upper limit of deforming force up to which, if deforming force is replaced, the object regains its real form completely and after that limit, if deforming force is increased, the object loses its characteristic of elasticity and gains permanently deformed.
Elastic limit is the property of a body whereas elasticity is the property of the material of a body.
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