A falling rock has kinetic energy KE just before striking the ground and coming to rest. What is the total change in entropy of rock plus environment as a result of this collision?
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This is Giancoli Answers with Mr. Dychko. The change in entropy of the rock and the environment, basically the change in entropy of the universe, is the change in entropy of the rock itself plus the change in entropy of its surrounding environment. Change in entropy of the rock will be the heat gained by the rock divided by its temperature. The rock and the environment both have the same temperature so there's no subscript to distinguish them. Both are just T. The change in entropy of the environment is the heat gained by the environment divided by T. They both have the same denominators so we can just go Q rock plus Q environment over T for the total change in entropy. All these heat came from the kinetic energy that the rock had initially. That's where the kinetic energy went to because the rock has stopped now, so the energy is considered. And so where did it go? It turned up the heat. Some of the heat went into the rocks, some went into the environment. Since the sum is the kinetic energy initially of the rock, that means the total change in entropy is the rock's initial kinetic energy divided by temperature.
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