Young's Modulus

Young's modulus (E) relates the STRESS and STRAIN in a solid (usually wire) using the following formula:

E = stress/strain = σ/ε

where σ = Force/Area and E = Change in Length/Length

Young's modulus has units of newtons per metre squared (Nm^-2) and is calculated only when the material is under elastic conditions, i.e., the force applied does not exceed the elastic limit and cause deformation.

Taken from Dictionary of Science

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In solid mechanics, Young's modulus (E) is a measure of the stiffness of an isotropic elastic material. It is also known as the Young modulus, modulus of elasticity, elastic modulus (though Young's modulus is actually one of several elastic moduli such as the bulk modulus and the shear modulus) or tensile modulus. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. This can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material.

Young's modulus is named after Thomas Young, the 19th century British scientist. However, the concept was developed in 1727 by Leonhard Euler, and the first experiments that used the concept of Young's modulus in its current form were performed by the Italian scientist Giordano Riccati in 1782 — predating Young's work by 25 years.

Taken from Wikipedia