College Physics

joule:

kilowatt-hour:

kinetic energy:

law of conservation of energy:

mechanical energy:

metabolic rate:

net work:

nonconservative force:

nuclear energy:

potential energy of a spring:

potential energy:

power:

radiant energy:

renewable forms of energy:

thermal energy:

useful work:

watt:

work-energy theorem:

work:

SI unit of work and energy, equal to one newton-meter

(kW ⋅ h)unit used primarily for electrical energy provided by electric utility companies

the energy an object has by reason of its motion, equal to^1

2

mv^2 for the translational (i.e., non-rotational) motion of an object of

massmmoving at speedv

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

the sum of kinetic energy and potential energy

the rate at which the body uses food energy to sustain life and to do different activities

work done by the net force, or vector sum of all the forces, acting on an object

a force whose work depends on the path followed between the given initial and final configurations

energy released by changes within atomic nuclei, such as the fusion of two light nuclei or the fission of a heavy nucleus

the stored energy of a spring as a function of its displacement; when Hooke’s law applies, it is given by the

expression^1

2

kx

2

wherexis the distance the spring is compressed or extended andkis the spring constant

energy due to position, shape, or configuration

the rate at which work is done

the energy carried by electromagnetic waves

those sources that cannot be used up, such as water, wind, solar, and biomass

the energy within an object due to the random motion of its atoms and molecules that accounts for the object's temperature

work done on an external system

(W) SI unit of power, with1 W = 1 J/s

the result, based on Newton’s laws, that the net work done on an object is equal to its change in kinetic energy

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

Section Summary

7.1 Work: The Scientific Definition

• Work is the transfer of energy by a force acting on an object as it is displaced.

• The workWthat a forceFdoes on an object is the product of the magnitudeFof the force, times the magnitudedof the displacement,

times the cosine of the angleθbetween them. In symbols,

W=Fdcosθ.

• The SI unit for work and energy is the joule (J), where1 J = 1 N ⋅ m = 1 kg ⋅ m

2

/s

2

.

• The work done by a force is zero if the displacement is either zero or perpendicular to the force.


• The work done is positive if the force and displacement have the same direction, and negative if they have opposite direction.

7.2 Kinetic Energy and the Work-Energy Theorem

• The net workWnetis 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 massmmoving at speedvisKE =^1

2

mv^2.

• The work-energy theorem states that the net workWneton a system changes its kinetic energy,Wnet=^1

2

mv^2 −^1

2

mv 02.

7.3 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,ΔPEg, isΔPEg=mgh, withhbeing the increase in height andgthe 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,ΔPEg, have physical significance.

254 CHAPTER 7 | WORK, ENERGY, AND ENERGY RESOURCES

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