Pythonが持参した小さなdemo(下)

!/usr/bin/env python3
"""turtle-example-suite:

    tdemo_planets_and_moon.py

Gravitational system simulation using theapproximation method from Feynman-lectures,p.9-8, using turtlegraphics.Example: heavy central body, light planet,very light moon!Planet has a circular orbit, moon a stableorbit around the planet.You can hold the movement temporarily bypressing the left mouse button with themouse over the scrollbar of the canvas."""from turtle import Shape, Turtle, mainloop, Vec2D as VecG = 8class GravSys(object):

def __init__(self):
    self.planets = []
    self.t = 0
    self.dt = 0.01
def init(self):
    for p in self.planets:
        p.init()
def start(self):
    for i in range(10000):
        self.t += self.dt
        for p in self.planets:
            p.step()

class Star(Turtle):

def __init__(self, m, x, v, gravSys, shape):
    Turtle.__init__(self, shape=shape)
    self.penup()
    self.m = m
    self.setpos(x)
    self.v = v
    gravSys.planets.append(self)
    self.gravSys = gravSys
    self.resizemode("user")
    self.pendown()
def init(self):
    dt = self.gravSys.dt
    self.a = self.acc()
    self.v = self.v + 0.5*dt*self.a
def acc(self):
    a = Vec(0,0)
    for planet in self.gravSys.planets:
        if planet != self:
            v = planet.pos()-self.pos()
            a += (G*planet.m/abs(v)**3)*v
    return a
def step(self):
    dt = self.gravSys.dt
    self.setpos(self.pos() + dt*self.v)
    if self.gravSys.planets.index(self) != 0:
        self.setheading(self.towards(self.gravSys.planets[0]))
    self.a = self.acc()
    self.v = self.v + dt*self.a

create compound yellow/blue turtleshape for planets
def main():

s = Turtle()
s.reset()
s.getscreen().tracer(0,0)
s.ht()
s.pu()
s.fd(6)
s.lt(90)
s.begin_poly()
s.circle(6, 180)
s.end_poly()
m1 = s.get_poly()
s.begin_poly()
s.circle(6,180)
s.end_poly()
m2 = s.get_poly()
planetshape = Shape("compound")
planetshape.[    ](https://www.gendan5.com/futures/cf.html)addcomponent(m1,"orange")
planetshape.addcomponent(m2,"blue")
s.getscreen().register_shape("planet", planetshape)
s.getscreen().tracer(1,0)
## setup gravitational system
gs = GravSys()
sun = Star(1000000, Vec(0,0), Vec(0,-2.5), gs, "circle")
sun.color("yellow")
sun.shapesize(1.8)
sun.pu()
earth = Star(12500, Vec(210,0), Vec(0,195), gs, "planet")
earth.pencolor("green")
earth.shapesize(0.8)
moon = Star(1, Vec(220,0), Vec(0,295), gs, "planet")
moon.pencolor("blue")
moon.shapesize(0.5)
gs.init()
gs.start()
return "Done!"

if name == '__main__':

main()
mainloop()