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re: frugal storm shelters?
9 may 2000
not believing a hurried, careless, and exaggerated report of the number
of casualties in the outer districts where trees, lamps, swans had been
reduced to clear fields of fire we issued entrenching tools to those
who seemed trustworthy and turned the heavy-weapons companies so that
we could not be surprised from that direction. we interrogated sojourner's
captured chinchilla . two of us forced his head back
while another poured water into his nostrils. his body jerked, he choked
and wept, but failed to answer the magical questions:
1. how many engineering degrees and years of experience do you have?
2. you talk about engineering, but do you know anything about engineering?
3. can you calculate how much distributed weight a 12' 2x4 can support?
with bending moment m = wl/8 = 12x12w/8 = 18w inch-pounds and modulus
s = m/f = 18w/1150 in^3 for eastern hem-fir, and bd^2 = 1.5x3.5^2 = 6s,
w = 196 pounds with the short face up. turned sideways, bd^2 = 7.88, so
it can only support a total of 84 pounds.
4. how much load will it support as a post?
with modulus of elasticity e = 1,400,000 psi and slenderness ratio
l/d = 12x12/1.5 = 96 and buckling load c = 0.3e/96^2 = 45.6 psi,
it can support 1.5x3.5c = 239 pounds as a post. a 4x4 with l/d = 41
can support 3039 pounds.
5. what's the chance of finding a working generator if two fail once
a month and a single repair person takes 4 hours to fix one, on average?
with failure rate l = 1/720 per hour and a 1/4h repair rate r,
2l l p2 <--> both work
---- --------> ---- -------> ---- p1 <--> one works
| p2 | | p1 | | p0 | p0 <--> none work
---- <-------- ---- <------- ----
r r
p2 + p1 + p0 = 1 and p1 = 2l/r p2 and p0 = l/r p1, so
p0 = 1/(1+r/l+r^2/(2l^2)) ~ 2x4^2/720^2 = 0.000062, ie
the chance of finding a working generator is 99.9938%, ie
they would both be down 2.67 minutes per month, on average.
adding a 3rd generator reduces the average system downtime
to 2.67 seconds per month.
6. how many volts do we need to force 10 amps of alternator
field current through 300k ohms of nichrome wire?
10a x 300kohms = 3 million volts. for further details, ask george.
7. what's the difference between cement and concrete?
aggregate. concrete also contains rocks, gravel, and so on.
nick
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