Question

Describe stress, strain, and elastic deformation, and give example

Answer 1

Stress:-When a deforming force is applied on a body then it changes the configuration(shape,size etc)of the body.When body deformed then positions of its atoms or molecules disturbed and shifted from their original positions.As a result an internal restoring force arises that tries to bring the body to its original position.

This internal restoring force per unit area of the body is called Stress.

                   Stress=Restoring Force/Area of cross section

It is seen that the internal restoring force by the atoms or molecules of the body at the time of deformation is equal to the external force applied on the body so we can write-

                 Stress=External Force/Area of cross section

its S.I. unit is N/m2= Nm^-2

Let we apply a 10N force on the end of a rod whose cross section area is 4cm².then Normal stress is given by-

Stress=force/cross section area=10/(4x10^-4)=2.5x10⁴ N/m²

----------------------------------------------------------------------------------------------------------------------------

Strain:-When a deforming force is applied to a body then its shape and size changed.The change in its configuration with respect to the original configuartion is called strain.

   Strain=Change in configuration/original configuration

if body is a thin rod and the deforming force changes its length (while change in its cross section area is neglegible) then for such kind of objects-

      Strain=change in length/original length

if body is a thin lamina shaped and the deforming force changes its area then for such kind of objects-

Strain=change in area/original area

If deforming force changes the violume of a body then-

Strain=change in volume/original volume

Strain has no unit.

example:-let we stretched a 1 meter long wire such that its new length becomes 1.02 meter then -

change in length=1.02-1=0.02m

Strain=change in length/original length =0.02/1=0.02

------------------------------------------------------------------------------------------------

Elastic deformation:-If a sufficeint force is applied on a body then its shape changes.This is called deformation.If on removing the external force body again recover its original shape then this deformation is called "Elastic deformation" otherwise it is called "Plastic deformation".

If the change in shape of a body is under iots limit of elasticity then body recoves its original shape.

Supposer we stretch a spring and release it then it quickly recover its original shape then this elongation is eleastic eleongation.But if we stretch the spring to a greater length then on releasing it will not recover its original shape so this time elongation is not elastic.