require 'tk'

c = TkCanvas.new.pack
TkcOval.new(c, 140, 0, 160, 20, 'fill'=>'red').addtag('item')
vx, vy = 0, 0
i = 0
TkAfter.new( 50, 24, proc{ c.move('item', vx, vy); i += 1; vy = 0.7*i }).start

Tk.mainloop