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Questions for Discussion 451

DEFORMATIONS OF ELASTIC MATERIAL
A is cross-sectional area.
¢L 1 F
Elongation: F (Y Young’s modulus) (14.18)
L Y A
Deformation is A
1 F
Shear: an elongation. L ¢x (S shear modulus) (14.19)
h S A
L
1 F
Compression: ¢V (B bulk modulus) (14.20)
V B A

A x
F
Deformation is h
a shear.

Deformation is
a compression.
A
F

QUESTIONS FOR DISCUSSION

1. If the legs of a table are exactly the same length and if the floor holding a load hangs from this rope. Explain why this device
is exactly flat, then the weight of the table will be equally dis- gives a very large mechanical advantage if the radii of the two
tributed over all four legs. But if there are small deviations from drums are nearly equal.
exactness, then the weight will not be equally distributed. Is it 5. The collapse of several skywalks at the Hyatt Regency hotel
possible for all of the weight to rest on three legs? On two? in Kansas City on July 17, 1982, with the loss of 114 lives, was
2. List as many examples as you can of joints in the human skele- due to a defective design of the suspension system. Instead of
ton that act as pivots for levers. Do any of these levers in the suspending the beams of the skywalks directly from single,
human skeleton have a mechanical advantage larger than 1? long steel rods anchored at the top of the building, some
3. Design a block and tackle with a mechanical advantage of 4, incompetent engineers decided to use several short steel rods
and another with a mechanical advantage of 5. If you connect joining the beams of each skywalk to those of the skywalk
these two arrangements in tandem, what mechanical advan- above (Fig. 14.31). Criticize this design, keeping in mind that
tage do you get? the beams are made of a much weaker material than the rods.
4. Figure 14.30 shows a differential windlass consisting of two
rigidly joined drums around which a rope is wound. A pulley

suspension
rod
beam of
skywalk

suspension
rod

FIGURE 14.31 Beam of skywalk
and suspension rods.
FIGURE 14.30
Differential windlass.
6. A steel rod is much less flexible than a woven steel rope of the
same strength. Explain this.