Now, let’s look at the roller coaster in Figure 9.7. In the discussions to follow, we will use the approximation that transformations are frictionless. Friction causes the loss of some useful energy. However, note that because of the work done by friction, these energy–work transformations are never perfect. If all the work done on an object is used to raise the object above the ground, the amount work equals the object’s gain in gravitational potential energy. This is generally true for any object raised above the ground. The amount of work required to raise the TV from point A to point B is equal to the amount of gravitational potential energy the TV gains from its height above the ground. Remember that both work and energy are expressed in joules. For example, when an object that has gravitational potential energy falls, its energy is converted to kinetic energy. You will see that this stored energy can either be used to do work or can be transformed into kinetic energy. Remember that the potential part of the term means that energy has been stored and can be used at another time. Quite a bit of potential energy is gained by a roller coaster car and its passengers when they are raised to the top of the first hill. We will also see that, in a closed system, the sum of these forms of energy remains constant. In this section we will see how energy is transformed from one of these forms to the other. We saw earlier that mechanical energy can be either potential or kinetic. Mechanical Energy and Conservation of Energy Explain that energy lost to friction is really transforming kinetic energy at the macroscopic level to kinetic energy at the atomic level. Try to get students to understand heat and temperature at a molecular level. Start a discussion about how other useful forms of energy also end up as wasted heat, such as light, sound, and electricity. Identify heat generated by friction as the usual explanation for apparent violations of the law. Discuss the law of conservation of energy and dispel any misconceptions related to this law, such is the idea that moving objects just slow down naturally. Explain how the general definition of energy as the ability to do work makes perfect sense in terms of either form of mechanical energy. Begin by distinguishing mechanical energy from other forms of energy. In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Work and Energy, as well as the following standards: (D) demonstrate and apply the laws of conservation of energy and conservation of momentum in one dimension.(B) investigate examples of kinetic and potential energy and their transformations.The student knows that changes occur within a physical system and applies the laws of conservation of energy and momentum. The learning objectives in this section will help your students master the following standards:
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