Unformatted text preview: (a) (b) (C) a «MamasE PROBLEM 3.8 It is known that a vertical force of 200 lb is required to remove the nail
at C from the board. As the nail ﬁrst starts moving, determine (a) the
moment about B of the force exerted on the nail, (b) the magnitude of
the force P that creates the same moment about B if a =10°, (c) the
smallest force P that creates the same moment about B. We have MB = rC,BFN
= (4 in.)(200 1b)
= 800 lb  in.
or MB =8001bin.)4
By definition M B = 7A,BP sin 9 6=lO°+ (180°~ 70°)
=120° Then 800 1b  in. = (18 in.) x Psin120°
or P=51.3 lb 4 For P to be minimum, it must be perpendicular to the line joining
Points A and B. Thus, P must be directed as shown. Thus M B = dem
d = rA/B
or 800 lb  in. = (18 in.)Pmin
or 13min =44.41b i"min =44.4 1171.200 4 ...
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 Fall '08
 Jenkins
 Statics

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