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

214 Chapter 8 Time and Time-Related Parameters

Figure 8.7. As the average speed of the vehicles increases, so does the flow of traffic, eventu-

ally reaching a maximum value. As the average speed of vehicles continues to increase, the flow

of traffic (the number of vehicles per hour) decreases. This region is marked by an uncon-

gested flow region.

Finally, traffic engineers use various measurement devices and techniques to obtain real-

time data on the flow of traffic. They use the collected information to make improvements to

move vehicles more efficiently. You may have seen examples of traffic measurement devices such

as pneumatic road tubes and counters. Other common traffic measurement devices include

magnetic induction loops and speed radar.

Example 8.2 Show that Equation (8.7) is dimensionally homogeneous by carrying out the appropriate units

for each term in that equation.

8.5 Engineering Parameters Involving Length and Time

In this section, we will considerderived physical quantitiesthat are based on the fundamental

dimensions of length and time. We will first discuss the concepts of linear speed and accelera-

tion and then define volumetric flow rate.

Linear Velocities

We will begin by explaining the concept of linear speed. Knowledge of linear speed and accel-

eration is important to engineers when designing conveyer belts that are used to load luggage

into airplanes and product assembly lines, treadmills, elevators, automatic walkways, escalators,

water or gas flow inside pipes, space probes, roller coasters, transportation systems (such as cars,

boats, airplanes, and rockets), snow removal equipment, backup computer tape drives, and so

on. Civil engineers are also concerned with velocities, particularly wind velocities, when design-

ing structures. They need to account for the wind speed and its direction in their calculations

when sizing structural members.

Let us now take a close look at what we mean by linear speed and linear velocity. All of you

are familiar with a car speedometer. It shows the instantaneous speed of a car. Before we explain

in more detail what we mean by the terminstantaneous speed, let us define a physical variable

that is more easily understood, theaverage speed, which is defined as

(8.8)

Note that the fundamental (base) dimensions of length and time are used in the definition

of the average speed. The average speed is called aderived physical quantitybecause its

average speed

distance traveled

time

q a

vehicles

hour

bk a

vehicles

kilometer

b u a

kilometer

hour

b

Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀