How Would I Make an Object Move by Itself?

For my simulation I am trying to make I need the shapes to have a gravity effect. In other words, when I click the button to create a new shape how would I make it fall down on its own? Any help would be appreciated.

Note: I've only been trying to get the circles to work for now so the other shape's code is behind. I also need to finish collision detection so that is why there is some random stuff for that in there.

main.py:

import pygame
import time
from shapes import *
from inv import *

pygame.init()

width, height = (1000, 800)
screen = pygame.display.set_mode((width, height))
pygame.display.set_caption("Physics Game")
bgc = (223, 255, 252)
screen.fill(bgc)

# Inv Button Variables
iwidth = 100
iheight = 100

# Inventory Classes
icir = InvCir(10, 10, iwidth, iheight)
irect = InvRect(15 + iwidth, 10, iwidth, iheight)
itri = InvTri(20 + (iwidth * 2), 10, iwidth, iheight)

# Object Classes
cir = Circle(40, (97, 160, 255), 500, 300)
rect = Rect(300, 300, 80, 80)

# Shape Lists
cirList = []
rectList = []

def main():
    run = True
    FPS = 60
    clock = pygame.time.Clock()

    def updateScreen():
        icir.draw(screen, (0, 0, 0))
        irect.draw(screen, (0, 0, 0))
        itri.draw(screen, (0, 0, 0))

        pygame.display.update()

    def addList(list, shape):
        list.append(shape)
        print(list)

    while run:
        clock.tick(FPS)

        mpos = pygame.mouse.get_pos()

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

            if event.type == pygame.MOUSEBUTTONDOWN:
                if icir.checkClick(mpos):
                    cir.itemDraw(screen)
                    addList(cirList, cir)


            if event.type == pygame.MOUSEBUTTONDOWN:
                if irect.checkClick(mpos):
                    rect.itemDraw(screen)
                    addList(rectList, rect)

        if pygame.mouse.get_pressed()[0]:
                pass

        

        updateScreen()

main()

shapes.py:

import pygame
import os
from inv import *

pygame.init()

bgc = (223, 255, 252)
width, height = (1000, 800)
screen = pygame.display.set_mode((width, height))

# Images
CIRCLE = pygame.image.load(os.path.join("assets", "circle.png"))

def collide(obj1, obj2):
    offsetX = obj2.x - obj1.x
    offsetY = obj2.y - obj1.y
    return obj1.mask.overlap(obj2.mask, (offsetX, offsetY)) != None

class Circle:
    def __init__(self, radius, color, x, y):
        self.radius = radius
        self.color = color
        self.x = x
        self.y = y
        self.img = CIRCLE
        self.mask = pygame.mask.from_surface(self.img)

    def itemDraw(self, window):
        window.blit(self.img, (self.x, self.y))

    def move(self, vel):
        self.y += vel

    def wallCol(self, height, width):
        pass

    def objCol(self, obj):
        return collide(self, obj)

class Rect:
    def __init__(self, x, y, width, height):
        self.x = x
        self.y = y
        self.width = width
        self.height = height
        self.color = (255, 163, 76)

    def itemDraw(self, window):
        pygame.draw.rect(window, self.color, (self.x, self.y, self.width, self.height))

inv.py:

import pygame
import os

pygame.init()

CIRCLE = pygame.image.load(os.path.join("assets", "circ_button.png"))
SQUARE = pygame.image.load(os.path.join("assets", "square_button.png"))
TRIANGLE = pygame.image.load(os.path.join("assets", "tri_button.png"))

class Inv:
    def __init__(self, x, y, width, height):
        self.x = x
        self.y = y
        self.width = width
        self.height = height

class InvCir(Inv):
    def __init__(self, x, y, width, height):
        super().__init__(x, y, width, height)

    def draw(self, window, outline = None):
        if outline:
            pygame.draw.rect(window, outline, (self.x - 2, self.y - 2, self.width + 4, self.height + 4), 0)

        window.blit(CIRCLE, (self.x, self.y))

    def checkClick(self, pos):
        if pos[0] > self.x and pos[0] < self.x + self.width:
            if pos[1] > self.y and pos[1] < self.y + self.height:
                return True

        return False

class InvRect(Inv):
    def __init__(self, x, y, width, height):
        super().__init__(x, y, width, height)

    def draw(self, window, outline = None):
        if outline:
            pygame.draw.rect(window, outline, (self.x - 2, self.y - 2, self.width + 4, self.height + 4), 0)

        window.blit(SQUARE, (self.x, self.y))
    
    def checkClick(self, pos):
        if pos[0] > self.x and pos[0] < self.x + self.width:
            if pos[1] > self.y and pos[1] < self.y + self.height:
                return True

        return False

class InvTri(Inv):
    def __init__(self, x, y, width, height):
        super().__init__(x, y, width, height)

    def draw(self, window, outline = None):
        if outline:
            pygame.draw.rect(window, outline, (self.x - 2, self.y - 2, self.width + 4, self.height + 4), 0)

        window.blit(TRIANGLE, (self.x, self.y))

    def checkClick(self):
        pass