Modern Engineering Thermodynamics

1.7 Classical Mechanical and Electrical Units Systems 13
Since the lbf, lbm, and s have the same meaning in both the new system and the traditional Engineering English units
system, it follows that
and that
1 chunk
s 2
= 32:174 ft
s 2
1 chunk = 32:174 ft = ð32:174 ftÞ
1m
= 9:806 m
3:281 ft
Exercises
7. Determine the weight at standard gravity of an object whose mass is 1.0 slug. Answer: Since force and weight are the
same, Eq. (1.8) gives F = W = mg/g c . From Table 1.2, we find that, in the Absolute English units system, g c = 1
(dimensionless). So the weight of 1.0 slug is W = (1.0 slug)(32.174 ft/s 2 )/1 = 32.174 slug (ft/s 2 ). But, from Eq. (1.8),
we see that 1.0 slug = 1.0 lbf·s 2 /ft, so the weight of 1 slug is then W = 32.174 (lbf·s 2 /ft)(ft/s 2 ) = 32.174 lbf.
8. Determine the mass of an object whose weight at standard gravity is 1 poundal. Answer: Using the same technique as in
Exercise 7, show that the mass of 1 poundal is m = Fg/g c = Wg/g c = (1 poundal)(1)/32.174 ft/s 2 = 0.03108 pdl·s 2 /ft =
0.03108 lbm.
9. W. H. Snedegar whimsically suggested the following new names for some of the SI units 6 :
1 far = 1 meter (m); 1 jog = 1 m/s; 1 pant = 1 m/s 2
1 shove = 1 newton (N); 1 grunt = 1 joule (J); 1 varoom = 1 watt (W)
1 lump = 1 kilogram (kg); 1 gasp = 1 pascal (Pa); 1 flab = 1kg·m 2
and so forth. Of course the Snedegar units would use the same unit prefixes as SI (see Table 1.5 later). For example, a
km would be a kilofar, a kJ would be a kilogrunt, a MPa would be a megagasp, and an incremental length (incremental
far) would probably be called a near. In this system the fundamental mass, length, and time (M, L, t) units are the
lump, far, and second. All other Snedegar units are secondary, being defined by some basic equation. For example, the
secondary unit for velocity, the jog, is defined from the definition of the dimensions of velocity as length per unit time
(L/t), or 1 jog = 1far/s.Thiscan,however,producesomeproblemsinusage.Inmechanics,theunitsofmicrostrain
would be microfar/far. Since a microfar is closer to a near than a far, microstrain units would probably become a near/
far. Such logistical inconsistency often adds confusion to an otherwise well-defined system of units.
Determine the relation between the primary and secondary Snedegar units for (a) force, (b) momentum (ML/t), (c) acceleration,
(d) work, (e) power, and (f) stress (F/L 2 ). Answers: (a)1shove= 1lump· far/s 2 ;(b)1lump· jog = 1 lump · far/s 2 ;
(c) 1 pant = 1far/s 2 ;(d)1grunt= 1 shove · far = lump · far 2 /s 2 ; (e) 1 varoom = 1grunt/s= shove· far/s = lump · far 2 /s 3 ;
(f) 1 gasp = shove/far 2 = 1 lump/far·s 2 .
6 Snedegar, W. H., “Letter to the Editor,” 1983. Am. J. Phys. 51, 684.
EXAMPLE 1.4
Time passes. You graduate from college and go on to become a famous NASA design engineer. You have sole responsibility for
the design and launch of the famous Bubble-II space telescope system. The telescope weighs exactly 25,000 lbf on the surface of
the Earth and is to be installed in an asynchronous Earth orbit with an orbital velocity of exactly 5000 mph (Figure 1.11).
a. What is the value of g c (in lbm·ft/lbf·s 2 ) in this orbit?
b. How much will the telescope weigh (in lbf) in Earth orbit where the local acceleration of gravity is only 2.50 ft/s 2 .
Weight = 25,000. lbf
V = 5000. mph
FIGURE 1.11
Example 1.4.
(Continued )