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

Problems 193

computed in this manner to your results in parts (a)

and (b). Are there any other ways that you could

have determined the area of the profile? Explain.

7. 
   
   
   
   20. Estimate how much material is needed to make 100,000
       traffic yield signs. Investigate the size of one side of a
       yield sign and the kind of material it is made from. Write
       a brief memo to your instructor discussing your findings.
       7.21. As explained in the chapter, the air resistance to motion
       of a vehicle is determined experimentally by placing it in
       a wind tunnel. The air speed inside the tunnel is changed,
       and the drag force acting on the vehicle is measured. The
       experimental data is normally represented by a single
       coefficient that is called thedrag coefficient. It is defined
       by the following relationship:
   
   
   
   

or, in a mathematical form,

It was also explained in this chapter that the frontal

areaAis the frontal projection of the 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 vehicle when you view it from a direc-

tion normal to the front windshield. The 0.85 factor

is to adjust for rounded corners, open space below

the bumper, and so on. Typical drag coefficient val-

ues for sports cars are between 0.27 and 0.38, and

for sedans the values are between 0.34 and 0.5. This

assignment requires you to actually measure the

frontal area of a car. Tape a ruler or a yardstick to the

bumper of your car. The ruler will serve as a scale.

Take a photograph of the car and use any of the

methods discussed in this chapter to compute the

frontal area of the car.

7. 
   
   
   
   22. A machinist in an engineering machine shop has
       ordered a sheet of plastic with dimensions of 10 ft 
       12 ft 1 in. width, length, and thickness, respecti-
       vely. Can the machinist get the sheet of plastic inside
       the machine shop provided that the door dimensions
       are 6 ft 8 ft? Give the maximum dimensions of a
       sheet that can be moved inside the shop and explain
       how.
   
   
   
   

Cd

Fd

1

2

rV

2

A

.



drag force

1

2

1 air density 2 1 air speed 2

2

1 frontal area 2

Problem 7.14

0 500 miles

7.1 5. Obtain a woman’s high-heeled dress shoe and a woman’s

athletic walking shoe. For each shoe, make imprints of the

floor contact surfaces. Determine the total area for each

shoe, and assuming the woman who wears these shoes

weighs 120 lb, calculate the average pressure at the bottom

of the shoe for each shoe style. What are your findings?

What are your recommendations? Recall that

7.16. Obtain two different brands of cross-country skis. Make

imprints of the expected contact surface of the skies with

snow. Obtain a value for the average pressure for each

ski, assuming the person wearing the ski weighs 180 lb.

7.1 7. Estimate the average pressure exerted on the road by

the following.

a. A family car

b. A truck

c. A bulldozer

Discuss your findings.

7.18. Using area as your variable, suggest ways to cool freshly

baked cookies faster.

7.1 9. Estimate how much material is needed to make 100,000

stop signs. Investigate the size of one side of a stop sign

and the kind of material it is made from. Write a brief

memo to your instructor discussing your findings.

pressure

force

area

drag

coefficient

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