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In the gravitational systems, the weight of the mass unit (pound-mass) on Earth's surface is approximately equal to the force unit (pound-force). This is convenient because one pound mass exerts one pound force due to gravity. Note, however, unlike the other systems the force unit is not equal to the mass unit multiplied by the acceleration unit[8]—the use of Newton's Second Law, F = m·a, requires another factor, gc, usually taken to be 32.174049 lbm·ft/lbf·s2 = 32.174049 lbm/slug. "Absolute" systems are coherent systems of units: by using the slug as the unit of mass, the "engineering" FPS system avoids the need for such a constant. The SI is an "absolute" metric system with kilogram and meter as base units.. D ]1 c: f y% I" r: N
) S6 h) s1 z/ v5 _ -----From Wikipedia
3 j6 [! O. u" @6 X* phttps://en.wikipedia.org/wiki/Pound_(force)#Definitions7 A, u8 g" a4 S# ^
" s4 }+ l2 [. [以上是关于磅的相关内容。. b3 t) h: `4 R, C& b( L3 q
: K# m, {+ D6 v7 Y听八爷讲当年俄国人造飞机，将单位改成米规英制，其实质是什么？
6 x0 ?, l: a( L* d' [, [; X4 ?$ v在材料力学方面，老美的教材要求学生掌握两种单位。
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8 _& j2 L: m( O! n2 Q7 `. e. iA Careful Balance Between SI and U.S. Customary Units Is1 k5 a# x( u1 O7 w7 \% V0 v
Consistently Maintained. Because it is essential that students be! C( T) N0 A/ l3 @/ m5 S
able to handle effectively both SI metric units and U.S. customary
+ N7 D& R& \' m: g6 y8 x Qunits, half the examples, sample problems, and problems to be
8 k% r5 M: I2 ~5 {assigned have been stated in SI units and half in U.S. customary4 l8 }) l1 ?) [" M; D H9 B
units. Since a large number of problems are available, instructors can
7 K: B; I. Y- K9 H+ b; ~assign problems using each system of units in whatever proportion
6 S& f7 l+ S% o, b% s sthey find most desirable for their class.0 M# l2 Y2 h) X( q) m' i8 ^
! j) u D. s X8 `0 f; g
——引自 MECHANICS of MATERIALS5 x2 z3 F/ d7 W P, _9 e
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发表于 2016-1-29 13:42:36
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