import graphics as gr
SIZE_X = 800
SIZE_Y = 800
window = gr.GraphWin("Model", SIZE_X, SIZE_Y)
coords = gr.Point(400, 700)
velocity = gr.Point(2, 0)
acceleration = gr.Point(0, 0)
def add(point_1, point_2):
    new_point = gr.Point(point_1.x + point_2.x,
                         point_1.y + point_2.y)
    return new_point
def sub (point_1, point_2):
    new_point = gr.Point(point_1.x - point_2.x,
                         point_1.y - point_2.y)
    return new_point
def clear_window():
    rectangle = gr.Rectangle(gr.Point(0, 0), gr.Point(SIZE_X, SIZE_Y))
    rectangle.setFill('black')
    rectangle.draw(window)
    sun = gr.Circle(gr.Point(400, 400), 50)
    sun.setFill('yellow')
    sun.draw(window)
def draw_ball(coords):
    circle = gr.Circle(coords, 10)
    circle.setFill('red')
    circle.draw(window)
    return circle
def check_coords(coords, velocity):
    if coords.x < 0 or coords.x > SIZE_X:
        velocity.x = -velocity.x
    if coords.y < 0 or coords.y > SIZE_Y:
        velocity.y = -velocity.y
def update_coords(coords, velocity):
    return add(coords, velocity)
def update_velocity(velocity, acceleration):
    return add(velocity, acceleration)
def update_acceleration(ball_coords, center_coords):
    diff = sub(ball_coords, center_coords)
    distance_2 = (diff.x ** 2 + diff.y ** 2) ** (3/2)
    G = 2000
    return gr.Point(-diff.x*G/distance_2, -diff.y*G/distance_2)
clear_window()
circle = draw_ball(coords)
diff_coords = coords
while True:
    acceleration = update_acceleration(coords, gr.Point(400, 400))
    coords = update_coords(coords, velocity)
    velocity = update_velocity(velocity, acceleration)
    check_coords(coords, velocity)
    circle.move(coords.x - diff_coords.x, coords.y - diff_coords.y)
    diff_coords = coords
    gr.time.sleep(0.01)