PyGame - Particle Objects and Classes

#!/usr/bin/env python

import pygame, sys, random
from pygame.locals import *

w = 800
h = 480

z = 0

screen = pygame.display.set_mode((w,h))

pygame.display.update()

class Ball:
    def __init__(self, radius, y, x, color, size, maxforce, force, life):
        self.y=y
        self.x=x
        self.size=size
        self.maxforce=maxforce
        self.force=force
        self.radius=radius
        self.color=color
        self.life=life
        pygame.draw.circle(screen, self.color, (self.x, self.y), self.radius)

    def fall(self):
        if self.y < h-self.radius:
            self.y+=self.force
            if self.force < self.maxforce:
                self.force+=1
        elif self.y > h-self.radius or self.y == h-self.radius:
            self.y = h-self.radius-1
            self.force = self.force*-1
            self.maxforce = self.maxforce/2
        pygame.draw.circle(screen, self.color, (self.x, self.y), self.radius)
        self.life-=1
        if self.life < 0:
            ball.remove(self)


clock = pygame.time.Clock()
ball = []
ball.append(Ball(25, 250, 250, (random.randint(1,255),random.randint(1,255),random.randint(1,255)),"L", 25, 1 ,100))

while 1:
    clock.tick(60)
    x,y = pygame.mouse.get_pos()
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()
        elif event.type == MOUSEBUTTONDOWN:
            z = 1
        elif event.type == MOUSEBUTTONUP:
            z = 0

    if z == 1:
        ball.append(Ball(25, y, x, (random.randint(1,255),random.randint(1,255),random.randint(1,255)),"L", 25, 1 ,100))
        z = 3
    elif z > 1:
        z-=1

    screen.fill((0,0,0))

    for i in ball:
        i.fall()

    pygame.display.update()