Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

13.1 Work, Mechanical Energy, Thermal Energy 373

The objectives of this chapter are to introduce the concept of energy, the various types

of energy, and what is meant by the term power. We will explain various mechanical

forms of energy including kinetic energy, potential energy, and elastic energy. We will

also revisit the definition of thermal energy forms from Chapter 11, including heat

and internal energy. Next, we will present conservation of energy and its applications.

We will define power as the rate of doing work and explain in detail the difference

between work, energy and power. After our discussion, these differences should be

clear to you. The common units of power, watts and horsepower, are also explained.

Once you have a good grasp of the concepts of work, energy, and power, then you can

better understand the manufacturer’s power ratings of engines and motors. Moreover,

in this chapter we will also explain what is meant by the term efficiency and look at

the efficiencies of power plants, internal combustion engines (car engines), electric

motors, pumps, and heating, air-conditioning, and refrigeration systems.

13.1 Work, Mechanical Energy, Thermal Energy

As we explained in Chapter 10, mechanical work is performed when a force moves an object

through a distance. But what is energy? Energy is one of those abstract terms that you already have

a good feel for. For instance, you already know that we need energy to create goods, to build shel-

ter, to cultivate and process food, and to maintain our living places at comfortable temperature

and humidity settings. But what you may not know is that energy can have different forms. Recall

that scientists and engineers define terms and concepts to explain various physical phenomena that

govern nature. To better explain quantitatively the requirements to move objects, to lift things,

to heat or cool objects, or to stretch materials, energy is defined and classified into different cat-

egories. Let us begin with the definition ofkinetic energy. When work is done on or against an

object, it changes the kinetic energy of the object (see Figure 13.1). In fact, as you will learn in

more detail in your physics or dynamics class, mechanical work performed on an object brings

about a change in the kinetic energy of the object according to

(13.1)

wheremis the mass of the object andV 1 andV 2 are the speed of the object at positions 1

and 2, respectively. To better demonstrate Equation (13.1), consider the following example.

When you push on a lawn mower, which is initially at rest, you perform mechanical work on

the lawn mower and move it, consequently changing its kinetic energy from a zero value to

some nonzero value.

work 1  2 

1

2

mV

2

2 

1

2

mV

2

1

Work done by

the engine

Position 1 Position 2

mV^2

1

2 1

mV 2

1

2

2

■Figure 13.1

The relationship between work

and change in kinetic energy.

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