make "angle 45
make "L 1
make "bob 10

to draw.pendulum
  clearscreen
  seth :angle+180		; down on screen is 180
  forward :L*100-:bob
  penup
  forward :bob
  pendown
  arc 360 :bob
end

make "G   9.80665
make "dt  1/30
make "acc 0
make "vel 0

to step.pendulum
  make "acc  -:G / :L * sin :angle
  make "vel   :vel   + :acc * :dt
  make "angle :angle + :vel * :dt
  wait :dt*60
  draw.pendulum
end

hideturtle
until [key?] [step.pendulum]