# pendulumΒΆ

Animation of a damped pendulum

from numpy import sin, cos, sqrt, pi, array
import time
import gr

def rk4(x, h, y, f):
k1 = h * f(x, y)
k2 = h * f(x + 0.5 * h, y + 0.5 * k1)
k3 = h * f(x + 0.5 * h, y + 0.5 * k2)
k4 = h * f(x + h, y + k3)
return x + h, y + (k1 + 2 * (k2 + k3) + k4) / 6.0

def damped_pendulum_deriv(t, state):
theta, omega = state
return array([omega, -gamma * omega - 9.81 / L * sin(theta)])

def pendulum(t, theta, omega, acceleration):
gr.clearws()
gr.setviewport(0, 1, 0, 1)

x = [0.5, 0.5 + sin(theta) * 0.4]
y = [0.8, 0.8 - cos(theta) * 0.4]
# draw pivot point
gr.fillarea([0.46, 0.54, 0.54, 0.46], [0.79, 0.79, 0.81, 0.81]),

gr.setlinecolorind(1)
gr.setlinewidth(2)
gr.polyline(x, y)               # draw rod
gr.setmarkersize(5)
gr.setmarkertype(gr.MARKERTYPE_SOLID_CIRCLE)
gr.setmarkercolorind(86)
gr.polymarker([x[1]], [y[1]])   # draw bob
gr.setlinecolorind(4)
V = 0.05 * omega                # show angular velocity
gr.drawarrow(x[1], y[1], x[1] + V*cos(theta), y[1] + V*sin(theta))
gr.setlinecolorind(2)
A = 0.05 * acceleration         # show angular acceleration
gr.drawarrow(x[1], y[1], x[1] + A*sin(theta), y[1] + A*cos(theta))

gr.settextfontprec(2, gr.TEXT_PRECISION_STRING)
gr.setcharheight(0.032)
gr.settextcolorind(1)
gr.textext(0.05, 0.95, 'Damped Pendulum')
gr.setcharheight(0.040)
gr.mathtex(0.4, 0.22, '\\omega=\\dot{\\theta}')
gr.mathtex(0.4, 0.1, '\\dot{\\omega}=-\\gamma\\omega-\\frac{g}{l}sin(\\theta)')
gr.setcharheight(0.028)
gr.textext(0.05, 0.22, 't:%7.2f' % t)
gr.textext(0.05, 0.16, '\\theta:%7.2f' % (theta / pi * 180))
gr.settextcolorind(4)
gr.textext(0.05, 0.10, '\\omega:%7.2f' % omega)
gr.settextcolorind(2)
gr.textext(0.05, 0.04, 'y_{A}:%6.2f' % acceleration)

gr.updatews()

theta = 70.0   # initial angle
gamma = 0.1    # damping coefficient
L = 1          # pendulum length

t = 0
dt = 0.04
state = array([theta * pi / 180, 0])

now = time.clock()

while t < 30:
start = now

t, state = rk4(t, dt, state, damped_pendulum_deriv)
theta, omega = state
acceleration = sqrt(2 * 9.81 * L * (1 - cos(theta)))
pendulum(t, theta, omega, acceleration)

now = time.clock()
if start + dt > now:
time.sleep(start + dt - now)