Chapter 5: Work, Power, and Energy
Glossary
5.1 Work
- energy
- the ability to do work
- work
- the transfer of energy by a force that causes an object to be displaced; the product of the component of the force in the direction of the displacement and the magnitude of the displacement
- joule
- SI unit of work and energy, equal to one newton-meter
5.2 Kinetic Energy and the Work-Energy Theorem
- net work
- work done by the net force, or vector sum of all the forces, acting on an object
- work-energy theorem
- the result, based on Newton’s laws, that the net work done on an object is equal to its change in kinetic energy
- kinetic energy
- the energy an object has by reason of its motion, equal to [latex]\frac{1}{2}{\text{mv}}^{2}[/latex] for the translational (i.e., non-rotational) motion of an object of mass [latex]m[/latex] moving at speed [latex]v[/latex]
5.3 Gravitational Potential Energy
gravitational potential energy
the energy an object has due to its position in a gravitational field
5.4 Conservative Forces and Potential Energy
- conservative force
- a force that does the same work for any given initial and final configuration, regardless of the path followed
- potential energy
- energy due to position, shape, or configuration
- potential energy of a spring
- the stored energy of a spring as a function of its displacement; when Hooke’s law applies, it is given by the expression [latex]\frac{1}{2}{\text{kx}}^{2}[/latex] where [latex]x[/latex] is the distance the spring is compressed or extended and [latex]k[/latex] is the spring constant
- conservation of mechanical energy
- the rule that the sum of the kinetic energies and potential energies remains constant if only conservative forces act on and within a system
- mechanical energy
- the sum of kinetic energy and potential energy
5.5 Nonconservative Forces
- nonconservative force
- a force whose work depends on the path followed between the given initial and final configurations
- friction
- the force between surfaces that opposes one sliding on the other; friction changes mechanical energy into thermal energy
thermal energy
the energy within an object due to the random motion of its atoms and molecules that accounts for the object’s temperature
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5.6 Conservation of Energy
- law of conservation of energy
- the general law that 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
- electrical energy
- the energy carried by a flow of charge
- chemical energy
- the energy in a substance stored in the bonds between atoms and molecules that can be released in a chemical reaction
- radiant energy
- the energy carried by electromagnetic waves
- nuclear energy
- energy released by changes within atomic nuclei, such as the fusion of two light nuclei or the fission of a heavy nucleus
- thermal energy
- the energy within an object due to the random motion of its atoms and molecules that accounts for the object’s temperature
- efficiency
- a measure of the effectiveness of the input of energy to do work; useful energy or work divided by the total input of energy
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5.7 Power
- power
- the rate at which work is done
- watt
- (W) SI unit of power, with [latex]\text{1 W}=\text{1 J/s}[/latex]
- horsepower
- an older non-SI unit of power, with [latex]\text{1 hp}=\text{746 W}[/latex]
- kilowatt-hour
- \(\left(\text{kW}\cdot h\right)[/latex] unit used primarily for electrical energy provided by electric utility companies
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5.8 Work, Energy and Power in Humans
- metabolic rate
- the rate at which the body uses food energy to sustain life and to do different activities
- basal metabolic rate
- the total energy conversion rate of a person at rest
- useful work
- work done on an external system