Elasticity (physics)

In physics, elasticity (or stretchiness) is the inherent property of a body which tries to regain its original shape after removal of its shape by applying any force on it. Example:- An eraser coming back to its original shape after being pressed.

Some important definitions

1) Deforming force- A force that changes or tends to change the physical dimensions of a body. 2) Restoring force- It is an internal force which tries to regain its original dimentions after removal of any deforming force. 3) Elastic body - A body that has property of elasticity. 4) Inelasticity(Plasticity)- An inherit property of a body which doesn't regain it's original shape after removal of any deforming force.

Explanation

In general, if a stress is applied to a wire, the strain will increase in proportion(see above illustration) until a certain point called the limit of proportionality is reached. This is in accordance with Hooke's law. Thereafter there is at first a slight increase in strain with increased load until a point L is reached. Point L is called elastic limit. Up to this point, the deformation of the specimen is elastic, i.e. when the stress is removed the specimen returns to its original length. Beyond point L there is permanent deformation when the stress is removed, i.e. the material has ceased to be elastic and has become plastic. In the plastic stages individual materials vary somewhat.In general,however,at a point B there is a sudden increase in strain with further increase of stress.This point B is called yield point. Beyond point C, the Breaking stress, the wire will snap(which occurs at point D). Point D is called Breaking point..

Transitions to inelasticity

Above a certain stress known as the elastic limit or the yield strength of an elastic material, the relationship between stress and strain becomes nonlinear. Beyond this limit, the solid may deform irreversibly, exhibiting plasticity. A stress-strain curve is one tool for visualizing this transition.

Furthermore, not only solids exhibit elasticity. Some non-Newtonian fluids, such as viscoelastic fluids, will also exhibit elasticity in certain conditions. In response to a small, rapidly applied and removed strain, these fluids may deform and then return to their original shape. Under larger strains, or strains applied for longer periods of time, these fluids may start to flow like a liquid, with some viscosity.

Hooke's law for a spring

In contemporary language, we'd say this as something like extension is directly proportional to force only we'd replace the words "extension" with the symbol (Δx), "force" with the symbol (F), and "is directly proportional to" with an equals sign (=) and a constant of proportionality (k). To show that the springy object was trying to return to its original state, we'd also tack a negative sign (−) in front. In other words, we'd write the equation.

F = − k Δx

Stress

It is force acting per unit area on a body.

Stress=F/A

Strain

It is the ratio of the elongation of the material due to the deforming force(Δl) to the original length of the substance(l)

Strain=Δl/l

Bulk Strain

The ratio of change in volume in the material due to the deforming force to the original volume.

Bulk strain = Δv/v₀

Shear strain

the ratio of relative displacement between the layers to the perpendicular distance from the reference point to the point of application of force.

Young's Modulus

The ratio of longitudinal strain to longitudinal stress.

stress/strain= constant(Y)

hence- Y=Fl/AΔl

Elastic fatigue

Due to repeated application of force, if a body looses the property then it is called to suffer from elastic fatigue.'

Elastic limit

The maximum value of stress within the body regain its original shape is called elastic limit.

See also

• Stiffness
• Elastic modulus
• Linear elasticity
• Pseudoelasticity
• Viscoelasticity
• Resilience
• Plasticity
• Ductility

References

Oxford Dictionary of science,1999,4th edition.ISBN 0-19-280098-1