while True:
    rate(200)
    ball.pos = ball.pos + (ball.p/ball.mass)*dt
    if not (side > ball.pos.x > -side):
        ball.p.x = -ball.p.x
    if not (side > ball.pos.y > -side):
        ball.p.y = -ball.p.y
    if not (side > ball.pos.z > -side):
        ball.p.z = -ball.p.z