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

254 Chapter 10 Force and Force-Related Parameters

The simple examples given in Figure 10.2 are used to show you the effects of a force’s magni-

tude, direction, and point of application on the way an object behaves. Think about the different

situations shown in Figure 10.2 and then in your own words explain the behavior of each bar

and compare it to the other cases shown.

Tendencies of a Force

Now that you understand the concept of force, let us examine the tendencies of externally applied

forces. The natural tendency of a force acting on an object, if unbalanced, will be to translate the

object ( move it along) and to rotate it. Moreover, forces acting on an object could squeeze or

shorten, elongate, bend, or twist the object. The amount by which the object will translate,

rotate, elongate, shorten, bend, or twist will depend on its support conditions, and material and

geometric properties (length, area, and area moment of inertia). Machine components, tools,

parts of the human body, and structural members are generally subjected to push – pull, bend-

ing, or twisting types of loading. In order to quantify these tendencies in engineering, we define

terms such as moment, work, impulse, pressure, and stress. Those of you who are studying to

become aerospace, civil, manufacturing, or mechanical engineers will take classes in basic

mechanics and mechanics of materials in which you will explore forces, stresses, and materials

behavior in more depth. The simple examples given in Figure 10.3 show the tendencies of a

force. In your own words explain the behavior of each case.

Units of Force

The newton (N) is the unit of force in SI. One newton is defined as the force that will acceler-

ate 1 kilogram of mass at a rate of 1 m/s

2

. This relationship is based on Newton’s law of motion.

(10.1)

In comparison, in the British Gravitational System of Units, 1 pound force will accelerate 1 slug

at a rate of 1 ft /s

2

, as given by

(10.2)

And 1 pound force is equal to 4.45 newtons (1 lbf4.448 N).

1 lbf  1 1 slug 21 1 ft/s

2

2

1 newton 1 1 kg 21 1 m/s

2

2 or 1N1kg#m/s

2

F

2 F

F

F

F

Magnitude

Direction Point of application

■Figure 10.2 Simple examples to demonstrate the effect of

magnitude, direction, and the point of application of a force on the

same object. Explain the behavior of each case.

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