8: Chapter 8 - Energy


• 8.1: Prelude to Work, Energy, and 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 con
• 8.2: Work- The Scientific Definition
Work is the transfer of energy by a force acting on an object as it is displaced. The work $$W$$ that a force $$F$$ does on an object is the product of the magnitude $$F$$ of the force, times the magnitude $$d$$ of the displacement, times the cosine of the angle $$\theta$$ between them. In symbols, $W = Fd \space cos \space \theta.$ The SI unit for work and energy is the joule (J), where $$1 \space J = 1 \space N \cdot m = 1 \space kg \space m^2/s^2$$. The work done by a force is zero if the
• 8.3: Kinetic Energy and the Work-Energy Theorem
The net work $$W_{net}$$ is the work done by the net force acting on an object. Work done on an object transfers energy to the object. The translational kinetic energy of an object of mass $$m$$ moving at speed $$v$$ is $$KE = \frac{1}{2}mv^2$$. The work-energy theorem states that the net work $$W_{net}$$ on a system changes its kinetic energy, $$W_{net} = \frac{1}{2}mv^2 - \frac{1}{2}mv_0^2$$.
• 8.4: Gravitational Potential Energy
Work done against gravity in lifting an object becomes potential energy of the object-Earth system. The change in gravitational potential energy $$\Delta PE_g$$, is $$\Delta PE_g = mgh$$, with $$h$$ being the increase in height and $$g$$ the acceleration due to gravity. The gravitational potential energy of an object near Earth’s surface is due to its position in the mass-Earth system. Only differences in gravitational potential energy, $$\Delta PE_g$$,  have physical significance. As an obje
• 8.5: Conservation of Energy
The law of conservation of energy states that the total energy is constant in any process. Energy may change in form or be transferred from one system to another, but the total remains the same. When all forms of energy are considered, conservation of energy is written in equation form as $KE_i + PE_i + W_{nc} + OE_i = KE_f + PE_f + OE_f ,$ where $$OE$$ is all other forms of energy besides mechanical energy.
• 8.6: Nonconservative Forces
A nonconservative force is one for which work depends on the path. Friction is an example of a nonconservative force that changes mechanical energy into thermal energy. Work $$W_{nc}$$ done by a nonconservative force changes the mechanical energy of a system. In equation form, $$W_{nc} = \Delta KE + \Delta PE$$ or, equivalently, $$KE_i + PE_i + W_{nc} = KE_f + PE_f .$$ When both conservative and nonconservative forces act, energy conservation can be applied and used to calculate motion in terms
• 8.7: Power
Power is the rate at which work is done, or in equation form, for the average power $$P$$ for work $$W$$ done over a time $$t$$, $$P = W/t$$. The SI unit for power is the watt (W), where $$1 \space W = 1 \space J/s$$. The power of many devices such as electric motors is also often expressed in horsepower (hp), where $$1\space hp = 746 \space W.$$
• 8.8: Work, Energy, and Power in Humans
The human body converts energy stored in food into work, thermal energy, and/or chemical energy that is stored in fatty tissue. The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate, and the corresponding rate when at rest is called the basal metabolic rate (BMR) The energy included in the basal metabolic rate is divided among various systems in the body, with the largest fraction going to the liver and spleen, and the brain.

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