def setup():
    size(900, 450)
    
# we will update position & velocity frame-by-frame,
# so we store them in these "global" variables
y = 1.49  # need to change this to make anything happen!
vy = 0.0

def draw():
    dt = 0.01
    k = 20.0
    m = 1.0
    g = 9.8
    Lrelaxed = 1.0
    global y, vy
    y = y + vy*dt
    Fy = m*g - k*(y-Lrelaxed)
    vy = vy + (Fy/m)*dt
    xbob = 0
    ybob = Lrelaxed + y
    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)