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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.
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https://en.wikipedia.org/wiki/Pound_(force)#Definitions8 i6 _, R/ f. y0 X. k
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以上是關于磅的相關內容。8 y2 i" W X# g3 [! Z" I
% ]( R5 x' p1 G" n; K3 e聽八爺講當年俄國人造飛機，將單位改成米規英制，其實質是什么？
+ K1 @. C/ N# F7 G6 n2 Q: T在材料力學方面，老美的教材要求學生掌握兩種單位。- i r2 N! _" h4 y. m( ?6 \
, ]0 Y/ Y* F4 N7 K# G9 z6 ?A Careful Balance Between SI and U.S. Customary Units Is
: Q; B9 ^5 b) l) @7 mConsistently Maintained. Because it is essential that students be g8 f5 \+ V. |& A6 A4 v8 n* g1 }# a
able to handle effectively both SI metric units and U.S. customary
# A* ? G2 c: K; Uunits, half the examples, sample problems, and problems to be }5 Y1 A; Y* y; i) |
assigned have been stated in SI units and half in U.S. customary. E: E8 r/ i- M6 ?; \6 G9 T
units. Since a large number of problems are available, instructors can$ b# ?, M; ~1 S& R$ {" Z
assign problems using each system of units in whatever proportion
/ X( S! P, q3 ethey find most desirable for their class.& J- c/ l1 R8 t) I) u0 j& O; Z! N
2 y. G1 W; k* W' O1 L: F" L——引自 MECHANICS of MATERIALS
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