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# animation example code: double_pendulum_animated.pyΒΆ

```# Double pendulum formula translated from the C code at
# http://www.physics.usyd.edu.au/~wheat/dpend_html/solve_dpend.c

from numpy import sin, cos, pi, array
import numpy as np
import matplotlib.pyplot as plt
import scipy.integrate as integrate
import matplotlib.animation as animation

G =  9.8 # acceleration due to gravity, in m/s^2
L1 = 1.0 # length of pendulum 1 in m
L2 = 1.0 # length of pendulum 2 in m
M1 = 1.0 # mass of pendulum 1 in kg
M2 = 1.0 # mass of pendulum 2 in kg

def derivs(state, t):

dydx = np.zeros_like(state)
dydx[0] = state[1]

del_ = state[2]-state[0]
den1 = (M1+M2)*L1 - M2*L1*cos(del_)*cos(del_)
dydx[1] = (M2*L1*state[1]*state[1]*sin(del_)*cos(del_)
+ M2*G*sin(state[2])*cos(del_) + M2*L2*state[3]*state[3]*sin(del_)
- (M1+M2)*G*sin(state[0]))/den1

dydx[2] = state[3]

den2 = (L2/L1)*den1
dydx[3] = (-M2*L2*state[3]*state[3]*sin(del_)*cos(del_)
+ (M1+M2)*G*sin(state[0])*cos(del_)
- (M1+M2)*L1*state[1]*state[1]*sin(del_)
- (M1+M2)*G*sin(state[2]))/den2

return dydx

# create a time array from 0..100 sampled at 0.05 second steps
dt = 0.05
t = np.arange(0.0, 20, dt)

# th1 and th2 are the initial angles (degrees)
# w10 and w20 are the initial angular velocities (degrees per second)
th1 = 120.0
w1 = 0.0
th2 = -10.0
w2 = 0.0

# initial state
state = np.array([th1, w1, th2, w2])*pi/180.

# integrate your ODE using scipy.integrate.
y = integrate.odeint(derivs, state, t)

x1 = L1*sin(y[:,0])
y1 = -L1*cos(y[:,0])

x2 = L2*sin(y[:,2]) + x1
y2 = -L2*cos(y[:,2]) + y1

fig = plt.figure()
ax = fig.add_subplot(111, autoscale_on=False, xlim=(-2, 2), ylim=(-2, 2))
ax.grid()

line, = ax.plot([], [], 'o-', lw=2)
time_template = 'time = %.1fs'
time_text = ax.text(0.05, 0.9, '', transform=ax.transAxes)

def init():
line.set_data([], [])
time_text.set_text('')
return line, time_text

def animate(i):
thisx = [0, x1[i], x2[i]]
thisy = [0, y1[i], y2[i]]

line.set_data(thisx, thisy)
time_text.set_text(time_template%(i*dt))
return line, time_text

ani = animation.FuncAnimation(fig, animate, np.arange(1, len(y)),
interval=25, blit=True, init_func=init)

#ani.save('double_pendulum.mp4', fps=15)
plt.show()
```

Keywords: python, matplotlib, pylab, example, codex (see Search examples)