Lecture 9: 10/04/11 note: text in red refers to the textbook, 9 th edition extended. 1. Work and Potential energy – section 8-2 Consider a mass under gravity. If a mass (initially at rest) is raised slowly under the influence of gravity, the net work done on the mass is zero, since the work done by the force that is raising the mass is the negative of the work done by gravity and the two cancel. The mass therefore started with zero kinetic energy and ended with zero kinetic energy. However, in its new position, it has the potential to gain kinetic energy from the gravitational force if released. We can therefore say that for the (mass+earth) system, the process of raising the mass (or moving it away from the earth) has increased the potential energy of the (mass+earth). A second similar example. Consider a mass at rest at the end of an unstretched spring. Imagine that a force stretches out the spring slowly so that the velocity of the mass remains small at all times. When the spring and mass are stretched, we can say that the (spring+mass) system has
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