def setup():
    size(900, 450)

def draw():
    t = 0.01*frameCount
    omega = 1.0
    amplitude = 0.5
    Lequilibrium = 2.0
    xbob = 0
    ybob = Lequilibrium + amplitude * cos(omega*t)
    xpixel_anchor = 0.5*width
    ypixel_anchor = 0.01*height
    scale = 100.0
    xpixel_bob = xpixel_anchor + scale*xbob
    ypixel_bob = ypixel_anchor + scale*ybob
    # clear the screen for each new frame
    background(200)
    # draw the bob
    rbob = 15
    ellipse(xpixel_bob, ypixel_bob, 2*rbob, 2*rbob)
    # draw the spring as a series of zig-zag lines
    nzigzag = 20
    for i in range(nzigzag):
        spring_top = ypixel_anchor
        spring_bottom = ypixel_bob - rbob
        dy = (spring_bottom-spring_top)/nzigzag
        xzig = xpixel_anchor - 20
        yzig = ypixel_anchor + i*dy
        xzag = xpixel_anchor + 20
        ymid = yzig + 0.5*dy
        yzag = yzig + dy
        line(xzig, yzig, xzag, ymid)
        line(xzag, ymid, xzig, yzag)