import sys, pygame
pygame.init()

size = width, height = 320, 700
speed = [0,0.5]
black = 0, 0, 0
g = 0.098
dt = 1e-1
screen = pygame.display.set_mode(size)
ball = pygame.image.load("g:/ball.gif")
ballrect = ball.get_rect()
while 1:
    for event in pygame.event.get():
        if event.type == pygame.QUIT: sys.exit()
    speed[1] = speed[1] + g
    ballrect=ballrect.move(speed)
    if ballrect.left<0 or ballrect.right>width:
        speed[0]=-speed[0]
    if ballrect.top<0 or ballrect.bottom>height:
        speed[1]=-speed[1]
    screen.fill(black)
    screen.blit(ball,ballrect)
    pygame.display.flip()