Just another update

Task/Animate-a-pendulum/J/animate-a-pendulum-1.j

@@ -2,7 +2,7 @@ require 'gl2 trig'
 coinsert 'jgl2'
 
 DT   =: %30      NB. seconds
-ANGLE=: 0.25p1   NB. radians
+ANGLE=: 0.45p1   NB. radians
 L    =: 1        NB. metres
 G    =: 9.80665  NB. ms_2
 VEL  =: 0        NB. ms_1
@@ -33,8 +33,7 @@ drawPendulum=: verb define
   width=. {. glqwh''
   ps=. (-: width) , 40
   pe=. ps + 280 <.@* (cos , sin) 0.5p1 + y    NB. adjust orientation
-  glrgb 91 91 91
-  glbrush''
+  glbrush glrgb 91 91 91
   gllines ps , pe
   glellipse (,~ ps - -:) 40 15
   glellipse (,~ pe - -:) 20 20

Task/Animate-a-pendulum/J/animate-a-pendulum-2.j

@@ -0,0 +1,52 @@
+require 'gl2 trig'
+coinsert 'jgl2'
+
+DT   =: %30      NB. seconds
+ANGLE=: 0.45p1   NB. radians
+L    =: 1        NB. metres
+G    =: 9.80665  NB. ms_2
+VEL  =: 0        NB. ms_1
+
+PEND=: noun define
+pc pend;pn "Pendulum";
+minwh 320 200; cc isi isigraph type flush;
+)
+
+pend_run=: verb define
+  wd PEND,'pshow'
+  wd 'timer ',":DT * 1000
+)
+
+pend_close=: verb define
+  wd 'timer 0; pclose'
+)
+
+sys_timer_z_=: verb define
+  recalcAngle_base_ ''
+  wd 'psel pend; set isi invalid'
+)
+
+pend_isi_paint=: verb define
+  drawPendulum ANGLE
+)
+
+recalcAngle=: verb define
+  accel=. - (G % L) * sin ANGLE
+  VEL  =: VEL + accel * DT
+  ANGLE=: ANGLE + VEL * DT
+)
+
+drawPendulum=: verb define
+  width=. {. glqwh''
+  ps=. (-: width) , 20
+  pe=. ps + 150 <.@* (cos , sin) 0.5p1 + y    NB. adjust orientation
+  glclear''
+  glbrush glrgb 91 91 91                      NB. gray
+  gllines ps , pe
+  glellipse (,~ ps - -:) 40 15
+  glrect 0 0, width, 20
+  glbrush glrgb 255 255 0                     NB. yellow
+  glellipse (,~ pe - -:) 15 15                NB. orb
+)
+
+pend_run''