How to make a collision in manim?

Question

How can I make an elastic collision in manim my code right now only move the bullet forward but not bounce back when the conditional statement is true what did I get wrong?

class Problem2(Scene):
    def construct(self):
        square = Square(side_length=3)
        string = Line(square.get_corner(
            UP), [0, 4, 0], buff=0)

        bullet = Circle(radius=0.2, fill_color=GREY, fill_opacity=1)
        bullet.shift(6*LEFT)

        def updatebullet(bullet, dt):
            bullet_vel = 2
            bullet.shift(bullet_vel*dt)

            if bullet.get_x() >= square.get_x():
                bullet_vel = -bullet_vel

        bullet.add_updater(updatebullet)

        self.play(FadeIn(square, string))
        self.play(FadeIn(bullet))
        self.wait(10)

Answer 1

class Problem2(Scene):
    def construct(self):
        square = Square(side_length=3)
        string = Line(square.get_corner(
            UP), [0, 4, 0], buff=0)

        bullet = Circle(radius=0.2, fill_color=GREY, fill_opacity=1)
        bullet.shift(6*LEFT+DOWN)
        self.flag = False
        

        def updatebullet(bullet: Mobject, dt):
            if bullet.get_x() >= square.get_x() or self.flag:
                bullet_vel = -2
                self.flag = True
            else:
                bullet_vel = 2
            bullet.shift(np.array([bullet_vel, 0, 0])*dt)

        bullet.add_updater(updatebullet)

        self.play(FadeIn(square, string))
        self.play(FadeIn(bullet))
        self.wait(10)

Your problem was that every time the function was called, bullet_vel was automatically assigned to 2, then the shifting was done, and then it was changed. That meant the change never took effect, because each time the function was called it would reset. All I have done in this code is moved the changing of the velocity before the movement of the bullet (I also added a flag so that it wouldn't get stuck alternating between positive and negative velocities)