How to add collision between platform and player? (pygame)

Question

How do i make it so that when the player touches the platform, he does not fall through? when the game starts the player falls onto the platform. But right now he just falls through. I have implemented things such as velocity, gravity, and acceleration.

import pygame as pg
import os
# create a variable for pg.math.Vector2
vec = pg.math.Vector2

TITLE = "Jumping 1"

WIDTH = 800
HEIGHT = 600

clock = pg.time.Clock()
FPS = 60

GREEN = (0, 255, 0)
LIGHTBLUE = (50, 200, 250)
RED = (255, 0, 0)

# Player properties
PLAYER_ACC = 0.5
PLAYER_FRICTION = -0.12

game_folder = os.path.dirname(__file__)
img_folder = os.path.join(game_folder, "img")

class Player(pg.sprite.Sprite):
    def __init__(self):
        pg.sprite.Sprite.__init__(self)
        self.image = pg.Surface((50, 50))
        self.image.fill(GREEN)
        self.rect = self.image.get_rect()
        self.rect.center = ((WIDTH / 2, HEIGHT / 2))
        # position, velocity, acceleration
        self.pos = vec(WIDTH / 2, HEIGHT / 2)
        self.vel = vec(0, 0)
        self.acc = vec(0, 0)

    def update(self):
        self.acc = vec(0, 0.5)
        keystate = pg.key.get_pressed()
        if keystate[pg.K_LEFT]:
            self.acc.x = -PLAYER_ACC
        if keystate[pg.K_RIGHT]:
            self.acc.x = PLAYER_ACC

        # apply friction
        self.acc.x += self.vel.x * PLAYER_FRICTION
        # equations of motion
        self.vel += self.acc
        self.pos += self.vel + 0.5 * self.acc
        # wrap around the sides of the screen
        if self.pos.x > WIDTH:
            self.pos.x = 0
        if self.pos.x < 0:
            self.pos.x = WIDTH

        self.rect.midbottom = self.pos

player = Player()

class Platform(pg.sprite.Sprite):
    def __init__(self):
        pg.sprite.Sprite.__init__(self)
        self.image = pg.Surface((WIDTH, 50))
        self.image.fill(RED)
        self.rect = self.image.get_rect()
        self.rect.center = ((WIDTH / 2, HEIGHT - 25))\

platform = Platform()

def game_loop():
    pg.init()

    screen = pg.display.set_mode((WIDTH, HEIGHT))
    pg.display.set_caption(TITLE)

    all_sprites = pg.sprite.Group()
    all_sprites.add(player, platform)
    running = True
    while running:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                running = False

        all_sprites.update()

        screen.fill(LIGHTBLUE)
        all_sprites.draw(screen)
        pg.display.flip()

        clock.tick(FPS)

    pg.quit()

game_loop()