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Chapter%206 - COLLEGE PHYSICS Part I Chapter 6 Work and...

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COLLEGE PHYSICS, Part I Chapter 6: Work and Energy Work Done by a Constant Force The Work-Energy Theorem and Kinetic Energy Gravitational Potential Energy Conservative and Nonconservative Forces The Conservation of Mechanical Energy Nonconservative Forces and the Work-Energy Theorem Power Other Forms of Energy and the Conservation of Energy Work Done by a Variable Force
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An overview of Energy So far we were dealing with situations when the force and the acceleration were constant. In the real world it is not always that these parameters are constant. What happens when the force and the acceleration are not constant? In these cases Newton’s laws of motion cannot be applied directly. Energy (from the Greek νέργεια, meaning "activity, operation“) is a scalar quantity, a property of objects and systems that can be defined as the ability to do work. There are many different forms of energy, including, but not limited to, kinetic, potential, thermal, gravitational, sound energy, light (e.g., solar) energy, wind energy, elastic, electromagnetic, chemical, and nuclear energy. While one form of energy may be transformed to another, within an isolated system the total energy remains the same. This is the principle of conservation of energy , first postulated in the early 19 th century.
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Forms of Mechanical Energy The energy associated with motion, position, and deformation of objects is called the mechanical energy . The two well known forms of mechanical energy are the kinetic energy and the potential energy . The kinetic energy is the energy of an object due to the object’s motion at speed v . where m is the object’s mass. So the kinetic energy is determined by the object’s mass and the speed of motion. 2 1 2 kinetic E mv = The energy can also be stored as a result of the object’s position (like a swimmer just before jumping into the water from the top of a rock) or elastic deformation (like a stretched rubber band). This kind of stored energy is called the potential energy . The stretched rubber band stores potential energy, and when released, this energy is converted to the kinetic energy allowing it to fly across the room. The energy that is stored in an elastic object upon stretching is called elastic potential energy . A missile has kinetic energy . A stretched rubber band has potential energy . A swimmer uses his potential energy t o jump into the water.
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The skateboarder at the top of the quarter pipe has certain potential energy. When he starts descending from the top edge of the quarter pipe, he speeds up downwards because of the gravitational pull and gains kinetic energy; his initial potential energy converts to kinetic energy. This kind of potential energy is called gravitational potential energy .
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