Changed ode folder to be named diffyq, since many of these are pde videos

active_projects/diffyq/solve_pendulum_ode_sample_code.py

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+import numpy as np
+
+# Physical constants
+g = 9.8
+L = 2
+mu = 0.1
+
+THETA_0 = np.pi / 3  # 60 degrees
+THETA_DOT_0 = 0  # No initial angular velocity
+
+# Definition of ODE
+def get_theta_double_dot(theta, theta_dot):
+    return -mu * theta_dot - (g / L) * np.sin(theta)
+
+
+# Solution to the differential equation
+def theta(t):
+    # Initialize changing values
+    theta = THETA_0
+    theta_dot = THETA_DOT_0
+    delta_t = 0.01  # Some time step
+    for time in np.arange(0, t, delta_t):
+        # Take many little time steps of size delta_t
+        # until the total time is the function's input
+        theta_double_dot = get_theta_double_dot(
+            theta, theta_dot
+        )
+        theta += theta_dot * delta_t
+        theta_dot += theta_double_dot * delta_t
+    return theta
+
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