%matplotlib
import numpy as np
from mpl_toolkits.mplot3d import Axes3D
import matplotlib.pyplot as plt
import matplotlib.animation as animation

theta = np.pi/4
l0 = 0.2
x = 3*l0/2
dt = 0.001
t = 0
k = 2
m = 1
theta1 = 0
x1 = 0
T, X, THETA = ([], [], [])
X1, THETA1 = ([], [])

while t<20:
    T.append(t)
    X.append(x)
    X1.append(x1)
    THETA.append(theta)
    THETA1.append(theta1)

    theta2 = 9.8*np.sin(theta)/x
    x2 = 9.8*np.cos(theta)-k*x/m
    theta = theta+theta1*dt
    theta1 = theta1+theta2*dt
    x = x+x1*dt
    x1 = x1+x2*dt
    t=t+dt

X = np.array(X)
X1 = np.array(X1)
THETA = np.array(THETA)
THETA1 = np.array(THETA1)

fig, ax = plt.subplots()
# ax.plot(-(X+l0)*np.sin(THETA), (X+l0)*np.cos(THETA))
ax.plot(T, X)
plt.show()