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Three springs and a mass go to twitter

Wed, 18 May 2022

Physics Animations

All good posts start with a tweet. How about this one?

I invented this problem at breakfast and it's remarkably difficult: a mass m inside an equilateral triangle is connected by springs. Find out how it will move.
I failed: could not eliminate z (not even with mentioned relationship), nor found a good coordinate system. Tbc.. pic.twitter.com/CmSgIOGVwP

— Maarten Mortier (@maartengm) May 17, 2022

Let’s solve this using Python. First, some drawings:

import numpy as np
import matplotlib.pyplot as plt
%matplotlib inline

degree = np.pi / 180.
anchors = np.array([[0, 0],
                    [1, 0],
                    [np.cos(60 * degree), np.sin(60 * degree)]])

def draw_scene(x, y, ax):
    for anchor in anchors:
        ax.plot([anchor[0], x],
                [anchor[1], y], "--k")
    ax.plot(anchors[:, 0], anchors[:, 1], 'o')
    ax.plot(x, y, 'o', ms=10)
    
fig, ax = plt.subplots()
draw_scene(0.5 * np.cos(30 * degree), 0.5 * np.sin(30 * degree), ax)

Let’s now introduce the equations that time-step the mass that is subject to the spring forces.

def spring_force(start, stop, k, l0):
    AB = stop - start
    l = np.linalg.norm(AB)
    return -k * (l - l0) * AB / l

spring_force(np.array([0, 0]), np.array([1, 0]), 10, 0.5)

array([-5., -0.])

def step(x, xdot, ks, l0s, anchors, dt):
    force = np.zeros((2,))
    for start, k, l0 in zip(anchors, ks, l0s):
        force += spring_force(start, x, k, l0)
    xdot += force * dt
    x += xdot * dt
    return x, xdot

x = np.array([0.5 * np.cos(30 * degree), 0.5 * np.sin(30 * degree)])
xdot = np.array([0, 0.])
step(x, xdot, ks=[1, 2, 3], l0s=[2, 4, 6], anchors=anchors, dt=0.1)

(array([0.36669359, 0.12431156]), array([-0.66319116, -1.25688438]))

Let’s try some trajectories.

x = np.array([0.8, 0.8])
xdot = np.array([0.9, 0.6])
trajectory = []
for _ in range(700):
    x, xdot = step(x, xdot, ks=[1, 2, 3], l0s=[0.6, 0.8, 0.5], anchors=anchors, dt=0.1)
    trajectory.append(x.copy())
    
trajectory = np.array(trajectory)

fig, ax = plt.subplots()
ax.plot(trajectory[:, 0], trajectory[:, 1])
draw_scene(x[0], x[1], ax)

This looks good. Let’s make an animation.

from IPython.display import HTML
import matplotlib.animation as manim

fig, ax = plt.subplots()

def func(frame):
    ax.cla()
    ax.plot(trajectory[:int(frame), 0], trajectory[:int(frame), 1])
    draw_scene(trajectory[int(frame), 0], 
               trajectory[int(frame), 1], ax)
    
    
animation = manim.FuncAnimation(fig, func, frames=np.linspace(0, 700, 100, dtype=int, endpoint=False))

plt.close(fig)
HTML(animation.to_jshtml())

Once Loop Reflect