September 2017 Update

Task/Animate-a-pendulum/OoRexx/animate-a-pendulum.rexx

@@ -0,0 +1,35 @@
+pendulum = .pendulum~new(10, 30)
+
+before = .datetime~new
+do 100         -- somewhat arbitrary loop count
+  call syssleep .2
+  now = .datetime~new
+  pendulum~update(now - before)
+  before = now
+  say " X:" pendulum~x " Y:" pendulum~y
+end
+
+::class pendulum
+::method init
+  expose length theta x y velocity
+  use arg length, theta
+  x = rxcalcsin(theta) * length
+  y = rxcalccos(theta) * length
+  velocity = 0
+
+::attribute x GET
+::attribute y GET
+
+::constant g -9.81   -- acceleration due to gravity
+
+::method update
+  expose length theta x y velocity
+  use arg duration
+  acceleration = self~g / length * rxcalcsin(theta)
+  durationSeconds = duration~microseconds / 1000000
+  x = rxcalcsin(theta, length)
+  y = rxcalccos(theta, length)
+  velocity = velocity + acceleration * durationSeconds
+  theta = theta + velocity * durationSeconds
+
+::requires rxmath library