Python - Spring #2 - (StackOverflow.com)

#!/usr/bin/env python3

#
# http://stackoverflow.com/questions/41862541/simple-physics-string
#

import pygame

# --- constants ---

WHITE = (255, 255, 255)
BLACK = (  0,   0,   0)

FPS = 60

# --- classes ---

class Node():

    def __init__(self, position, mass=100, velocity=None):

        # default value for velocity,
        # list can't be used directly in `def`
        if velocity is None:
            velocity = [0, 1]

        self.position = pygame.math.Vector2(position)
        self.mass = mass
        self.velocity = pygame.math.Vector2(velocity)

    def update(self):
        self.position += self.velocity

class String():

    def __init__(self, nodes, gravity=.981, spring_constant=10, iterations=1):
        self.nodes = nodes
        self.gravity = gravity
        self.spring_constant = spring_constant
        self.iterations = iterations
        self.set_distance = 1

    def calculateForces(self):
        for x in range(self.iterations):
            for i in range(1, len(self.nodes)):

                previous = self.nodes[i-1]
                current = self.nodes[i]

                dxy = current.position - previous.position

                #distance = current.position.distance_squared_to(previous.position)
                distance = dxy.distance_squared_to((0,0))

                force = -self.spring_constant * (distance - self.set_distance)
                force = force / current.mass

                nDistanceVector = dxy * force / distance

                current.velocity = 0.71 * current.velocity + nDistanceVector

                current.velocity.y += self.gravity

                current.update()

# --- main ---

# - init -

pygame.init()

screen = pygame.display.set_mode((500, 500))

# - objects -

a = [Node([250 + i * 10, 100], 140) for i in range(25)]
s = String(a)

# - mainloop -

clock = pygame.time.Clock()

while True:

    # - events -

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            quit()

    # - updates -

    s.calculateForces()
    points = [node.position for node in a]

    # - draws -

    screen.fill(WHITE)
    pygame.draw.aalines(screen, BLACK, False, points)
    pygame.display.update()

    # - FPS -

    clock.tick(FPS)