College Physics

# Introduction to Work, Energy, and Energy Resources

College PhysicsIntroduction to Work, Energy, and Energy Resources
Figure 7.1 How many forms of energy can you identify in this photograph of a wind farm in Iowa? (credit: Jürgen from Sandesneben, Germany, Wikimedia Commons)

### Chapter Outline

7.1 Work: The Scientific Definition
• Explain how an object must be displaced for a force on it to do work.
• Explain how relative directions of force and displacement determine whether the work done is positive, negative, or zero.
7.2 Kinetic Energy and the Work-Energy Theorem
• Explain work as a transfer of energy and net work as the work done by the net force.
• Explain and apply the work-energy theorem.
7.3 Gravitational Potential Energy
• Explain gravitational potential energy in terms of work done against gravity.
• Show that the gravitational potential energy of an object of mass $mm size 12{m} {}$ at height $hh size 12{h} {}$ on Earth is given by $PEg=mghPEg=mgh$.
• Show how knowledge of the potential energy as a function of position can be used to simplify calculations and explain physical phenomena.
7.4 Conservative Forces and Potential Energy
• Define conservative force, potential energy, and mechanical energy.
• Explain the potential energy of a spring in terms of its compression when Hooke’s law applies.
• Use the work-energy theorem to show how having only conservative forces implies conservation of mechanical energy.
7.5 Nonconservative Forces
• Define nonconservative forces and explain how they affect mechanical energy.
• Show how the principle of conservation of energy can be applied by treating the conservative forces in terms of their potential energies and any nonconservative forces in terms of the work they do.
7.6 Conservation of Energy
• Explain the law of the conservation of energy.
• Describe some of the many forms of energy.
• Define efficiency of an energy conversion process as the fraction left as useful energy or work, rather than being transformed, for example, into thermal energy.
7.7 Power
• Calculate power by calculating changes in energy over time.
• Examine power consumption and calculations of the cost of energy consumed.
7.8 Work, Energy, and Power in Humans
• Explain the human body’s consumption of energy when at rest vs. when engaged in activities that do useful work.
• Calculate the conversion of chemical energy in food into useful work.
7.9 World Energy Use
• Describe the distinction between renewable and nonrenewable energy sources.
• Explain why the inevitable conversion of energy to less useful forms makes it necessary to conserve energy resources.

Energy plays an essential role both in everyday events and in scientific phenomena. You can no doubt name many forms of energy, from that provided by our foods, to the energy we use to run our cars, to the sunlight that warms us on the beach. You can also cite examples of what people call energy that may not be scientific, such as someone having an energetic personality. Not only does energy have many interesting forms, it is involved in almost all phenomena, and is one of the most important concepts of physics. What makes it even more important is that the total amount of energy in the universe is constant. Energy can change forms, but it cannot appear from nothing or disappear without a trace. Energy is thus one of a handful of physical quantities that we say is conserved.

Conservation of energy (as physicists like to call the principle that energy can neither be created nor destroyed) is based on experiment. Even as scientists discovered new forms of energy, conservation of energy has always been found to apply. Perhaps the most dramatic example of this was supplied by Einstein when he suggested that mass is equivalent to energy (his famous equation $E=mc2E=mc2 size 12{E="mc" {rSup} { size 8{2} } } {}$).

From a societal viewpoint, energy is one of the major building blocks of modern civilization. Energy resources are key limiting factors to economic growth. The world use of energy resources, especially oil, continues to grow, with ominous consequences economically, socially, politically, and environmentally. We will briefly examine the world’s energy use patterns at the end of this chapter.

There is no simple, yet accurate, scientific definition for energy. Energy is characterized by its many forms and the fact that it is conserved. We can loosely define energy as the ability to do work, admitting that in some circumstances not all energy is available to do work. Because of the association of energy with work, we begin the chapter with a discussion of work. Work is intimately related to energy and how energy moves from one system to another or changes form.

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