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College Physics 10.1

College Physics 10.1 - 10.13.2008 10.1 Elastic Deformations...

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10.13.2008 10.1 Elastic Deformations of Solids Object in equilibrium Zero net force Zero net torque Does not mean force and torques do no effect to object Object is deformed when contact forces are applied to it Deformation Change in size or shape of object Many solids are too stiff to see deformation with eye Require microscope or other device to detect When contact forces are removed Elastic object returns to its original shape and size Many objects are elastic as long as deforming forces are not too large Other hand, many objects can be permanently deformed or broken if forces are too large Ie. Car crash 10.2 Hooke’s Law of Tensile and Compressive Forces If stretched by same tensile forces and two wires get longer by proportional amount to their original lengths then they have same fractional length change or strain. Strain is dimensionless measure of degree of deformation Strain = delta L / L The force per unit area produces the same deformation on wires of same length and composition. The force per unit area is stress. Stress = F/A SI units are same as pressure: N/m^2 or Pa F = k delta L Rewrite: stress proportional to strain F/A = Y delta L/L
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Y is the proportionality constant and only depends on the inherent stiffness K = Y A/L Y is called elastic modulus or Young’s modulus It has the same units as Pa or N/m^2, same as stress Measures resistance of material to elongate or compress Flexible and stretches easily such as rubber has low Y Steel has high Y Hooke’s law holds up to maximum stress called proportional limit. 10.3 Beyond Hooke’s Law If tensile or compressive stress exceeds proportional limit, strain is no longer proportional to stress. The solid returns to original length when stress is removed as long as stress does not exceed elastic limit. If exceeded, material is permanently deformed. When stress reaches breaking point, the solid fractures. The maximum stress that can be withstood without breaking is called ultimate strength. Ultimate strength can be diff for compressive or tensile strength of material A ductile material continues to stretch beyond its ultimate tensile strength without breaking. The stress then decreases from ultimate strength.
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