UNIT OF STRESS
(i) N/m2 in S.I. System
.(ii) Dyne/cm2 in C.G.S. System.
STRAIN
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"Strain is the fractional deformation produced in a body when it is subjected to a set of deforming forces."
OR
The change in length, volume or shape due to the application of a deforming force is called "Strain".
OR
"The fractional Deformation due to stress is called STRAIN."
Strain has no unit because it is a ratio of two similar physical quantities.
LONGITUDINAL STRAIN
"The change in length per unit length of a body due to a deforming force is called LONGITUDINAL STRAIN"
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Mathematically:
HOOK'S LAW
Hook¢s law is a quantitative relation between stress and strain.
STATEMENT
Hook's law states:
"Within elastic limit , strain produced in a body is directly proportional to stress"
i.e.
Strain µ Stress
OR
Strain =(Constant)(Stress)
Where 1/K = constant of proportionality

Strain =(1/K)(Stress)

OR
Stress / Strain = K
OR
Stress / Strain = K
With the help of above equation Hook¢s law can also be stated as :
"The ratio of stress to strain is constant."
YOUNG'S MODULUS
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"The ratio of stress to the longitudinal strain is called Young's modulus."
Young's Modulus = Stress / Longitudinal Strain
OR
Y = s / e .......... (i)
But s = F / A and e = DL / L
Putting these values in equation (i)
Y =( F / A) / (DL / L)
OR
Y = F L / A DL
In S.I. system unit of ‘Young's Modulus is N/m2
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