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.PS
# FourbarSVG.m4
# https://tex.stackexchange.com/questions/563831/how-to-draw-four-bar-linkage-with-center-of-mass
gen_init
scale = 25.4
a = 18
b = 73
c = 47
d = 72
A0: Here
B0: A0+(d,0)
circlerad = 1
define(`pivot',`[
C: circle
line down_ 4 from C+(2,0)
arc from last line.start to C+(-2,0) with .c at C
line down_ 4
B: line thick 1.6 right 7 with .c at (C,Here)
thinlines_
sep = 0.9
nhash = B.len/sep
#for i=0 to nhash do { line down sep left sep from B.start+(i/nhash*B.len,0) }
for i=0 to nhash do { line down sep left sep from B.end-(i/nhash*B.len,0) }
thicklines_
]')
pivot with .C at A0; "svg_it(A`'svg_sub(0))" at A0-(5,0)
circle dashed rad a at A0
pivot with .C at B0; "svg_it(B`'svg_sub(0))" at B0-(5,0)
"svg_it(d = A`'svg_sub(0)B`'svg_sub(0))" at 0.5<A0,B0>
B1: Cintersect(A0,a+b,B0,c)
B2: Cintersect(A0,b-a,B0,c)
arc dashed from B1 to B2 with .c at B0
A0B1: line thick 1.6 from A0 to B1 chop 2 chop
line thick 1.6 from B0 to B1 chop 2 chop "svg_it(c)" ljust
circle fill_(1) at B1; "svg_it(B`'svg_sub(1))" wid 3 at B1+(5,0)
A1: circle fill_(1) at LCintersect(A0B1,A0,a,R)
"svg_it(A`'svg_sub(1))" at A1+(5,0)
A2: b/(b-a) between B2 and A0; "svg_it(a)" at 0.4<A2,A0> above rjust
"svg_it(b)" at 0.4<A1,B1> above rjust
A2B2: line thick 1.6 from A2 to B2
line thick 1.6 from B0 to B2 chop 2 chop 0
circle fill_(1) at B2; "svg_it(B`'svg_sub(2))" at B2+(5,0)
circle fill_(1) at A2; "svg_it(A`'svg_sub(2))" at A2+(5,0)
thinlines_
line from B0+(3,0) right 4
ArcAngle(Here,B0,B1,5) ->; "svg_psi`'svg_sub(0)" at last arc.ne above ljust
ArcAngle(B1,B0,B2,8) ->; "svg_psi" at last arc.n+(-3,0) above
ArcAngle(A1,A0,B2,b*0.45) ->; "svg_theta" at last arc.start+(0,4)
line from A0+(3,0) right 8
ArcAngle(Here,A0,B1,9) -> ; "svg_theta`'svg_sub(0)" at last arc.start+(2,2)
.PE
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