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

454 Chapter 14 Electronic Spreadsheets

14.9. Using Excel, create a table that shows the relationship

between the units of pressure in Pa and psi in the range

of 10 kPa to 100 kPa. Use increments of 5 kPa.

14.10. Using Excel, create a table that shows the relationship

between the units of power in watts and horsepower in

the range of 100 W to 10,000 W. Use smaller incre-

ments of 100 W up to 1000 W, and then use incre-

ments of 1000 W all the way up to 10,000 W.

14.11. As we explained in Chapter 7, the air resistance to the

motion of a vehicle is something important that engi-

neers investigate. As you may also know, the drag force

acting on a car is determined experimentally by placing

the car in a wind tunnel. The air speed inside the tunnel

is changed, and the drag force acting on the car is mea-

sured. For a given car, the experimental data is generally

represented by a single coefficient that is called thedrag

coefficient. It is defined by the following relationship:

where

Cddrag coefficient (unitless)

Fdmeasured drag force (N or lb)

rair density (kg /m

3

or slugs /ft

3

)

Vair speed inside the wind tunnel ( m/s or ft /s)

Afrontal area of the car ( m

2

or ft

2

)

Cd

Fd

1

2

rV

2

A

The frontal areaArepresents the frontal projection

of the car’s area and could be approximated simply

by multiplying 0.85 times the width and the height

of a rectangle that outlines the front of a car. This is

the area that you see when you view the car from a

direction normal to the front grills. The 0.85 factor

is used to adjust for rounded corners, open space

below the bumper, and so on. To give you some

idea, typical drag coefficient values for sports cars

are between 0.27 to 0.38, and for sedans are

between 0.34 to 0.5.

The power requirement to overcome air resistance

is computed by

where

Ppower (watts or ft#lb/s)

1 horsepower (hp) 550 ft#lb/s

and

1 horsepower (hp) 746 W

The purpose of this exercise is to see how the

power requirement changes with the car speed and

the air temperature. Determine the power require-

ment to overcome air resistance for a car that has a

listed drag coefficient of 0.4 and width of 74.4 in.

and height of 57.4 in. Vary the air speed in the range

PFdV

Problem 14.11

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