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S E C T I O N 1 2 . 4 • Elastic Properties of Solids 373 12.4 Elastic Properties of Solids Except for our discussion about springs in earlier chapters, we have assumed that ob- We shall discuss the deformation of solids in terms of the concepts of stress and strain. Stress is a quantity that is proportional to the force causing a deformation; more specifically, stress is the external force acting on an object per unit cross-sec- strain, which is a measure of the degree of defor- mation. It is found that, for sufficiently small stresses, strain is proportional to stress; the constant of proportionality depends on the material being deformed and elastic modulus. The elastic modulus is therefore defined as the ratio of the stress to the re- (12.5) The elastic modulus in general relates what is done to a solid object (a force is ap- We consider three types of deformation and define an elastic modulus for each: Elastic modulus $ stress strain Young’s Modulus: Elasticity in Length Consider a long bar of cross-sectional area A and initial length L i that is clamped at one end, as in Figure 12.14. When an external force is applied perpendicular to the f is greater than L i and in which the external force is exactly balanced by internal forces. In such a situation, the tensile stress as the ratio of the magnitude of the external force F to the cross-sectional area A. The tensile strain in this case is de- fined as the ratio of the change in length .L to the original length L i . We define Young’s modulus by a combination of these two ratios: (12.6) Y $ tensile stress tensile strain ! F/A ∆L/L i 1. Young’s modulus, which measures the resistance of a solid to a change in its length 2. Shear modulus, which measures the resistance to motion of the planes within a solid parallel to each other 3. Bulk modulus, which measures the resistance of solids or liquids to changes in their volume F A L i ∆L Active Figure 12.14 A long bar clamped at one end is stretched by an amount .L under the action of a force F. At the Active Figures link at http://www.pse6.com, you can adjust the values of the applied force and Young’s modulus to observe the change in length of the bar. break suddenly.) On the basis of symmetry, we assert that CB ! F CD and F CA ! F CE : Finally, we balance the horizontal forces on B, assuming that F CB ! 7 200 N !
F y ! 2 F CB sin 30* ' 7 200 N ! 0 Thus, the top bar in a bridge of this design must be very F BD ! 12 000 N (7 200 N) cos 30* $ (7 200 N) cos 30* ' F BD ! 0 ! F x ! F BA cos 30* $ F BC cos 30* ' F BD ! 0 Young’s modulus |