2013-3-1 Spring Work Substituting and integrating yield 6Wsping =F dx Wspring=)(kJ) sp6esanesnialandnefnai F=kx (kN)k:spring constant (kN/m) Elor ofa spring 2 mr position rce 600N is doubled Sign convention for Work Since engineering thermodynamics is often concerned with internal combustion engines, turbines,and electric generators whose purpose is to do work,it is convenient to regard the work done by a system as positive. >0:work done by the system <0:work done on the system The same sign convention is used for the rate of energy transfer by work-called power, denoted by w. Show direction of energy transfer by arrow 2-28 14 2013-3-1 14 27 Spring Work Elongation of a spring under the influence of a force. When the length of the spring changes by a differential amount dx under the influence of a force F, the work done is For linear elastic springs, the displacement x is proportional to the force applied k: spring constant (kN/m) Substituting and integrating yield x1 and x2: the initial and the final displacements The displacement of a linear spring doubles when the force is doubled. Sign convention for Work ►Since engineering thermodynamics is often concerned with internal combustion engines, turbines, and electric generators whose purpose is to do work, it is convenient to regard the work done by a system as positive. ►W > 0: work done by the system ►W < 0: work done on the system The same sign convention is used for the rate of energy transfer by work – called power, denoted by Show direction of energy transfer by arrow W. & 2-28