UNIT OF STRESS  
(i) N/m^{2} in S.I. System .(ii) Dyne/cm^{2} 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/m^{2}  
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