8.5 Engineering Parameters Involving Length and Time 215
definition is based on the fundamental dimensions of length and time. The SI unit for speed
is m/s, although for fast-moving objects km/h is also commonly used. In U.S. Customary
units, ft /s and miles /h ( mph) are used to quantify the magnitude of a moving object. Now
let us look at what we mean by the instantaneous speed and see how it is related to the aver-
age speed.
To understand the difference between average and instantaneous speed, consider the
following mental exercise. Imagine that you are going from New York City to Boston, a dis-
tance of 220 miles (354 km). Let us say that it took you 4.5 hours to go from the outskirts
of New York City to the edge of Boston. From Equation (8.8), you can determine your
average speed, which is 49 mph (79 km/h). You may have made a rest stop somewhere to
get a cup of coffee. Additionally, the posted highway speed limit may have varied from
55 mph (88 km/h) to 65 mph (105 km/h), depending on the stretch of highway. Based
on the posted speed limits and other road conditions, and how you felt, you may have
driven the car faster during some stretches, and you may have gone slower during other
stretches. These conditions led to an average speed of 49 mph (79 km/h). Let us also imag-
ine that you recorded the speed of your car as indicated by the speedometer every second.
The actual speed of the car at any given instant while you were driving it is called the
instantaneous speed.
To better understand the difference between the average speed and the instantaneous speed,
ask yourself the following question. If you needed to locate the car, would you be able to locate
the car knowing just the average speed of the car? The knowledge of the average speed of the
car would not be sufficient. To know where the car is at all times, you need more information,
such as the instantaneous speed of the car and the direction in which it is traveling. This means
you must know the instantaneous velocityof the car. Note that when we say velocity of a car,
we not only refer to the speed of the car but also the direction in which it moves.
Physical quantities that possess both a magnitude and a direction are called vectors. You
will learn more about vectors in your calculus, physics, and mechanics classes. For now, just
remember the simple definition of a vector quantity — a quantity that has both magnitude and
direction. A physical quantity that is described only by a magnitude is called a scalarquantity.
Examples of scalar quantities include temperature, volume, and mass. Examples of the range of
speed of various objects are given in Table 8.2.TABLE 8.2 Examples of Some Speeds
Situation m/s km/h ft /s mph
Average speed of a person walking 1.3 4.7 4.3 2.9
The fastest runner in the world (100 m) 10.2 36.7 33.5 22.8
Professional tennis player serving a ball 58 209 190 130
Top speed of a sports car 67 241 220 150
777 Boeing airplane (cruise speed) 248 893 814 555
Orbital speed of the space shuttle 7741 27,869 25,398 17,316
Average orbital speed of the earth 29,000 104,400 95,144 64,870
around the sunCopyright 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