require 'tk'

c = TkCanvas.new.pack

TkcOval.new(c, 40, 160, 60, 180, 'fill'=>'red').addtag('item')
def throw( c )
  c.delete('item')
  TkcOval.new(c, 40, 160, 60, 180, 'fill'=>'red').addtag('item')
  vx0, vy0 = 5, -14
  vx, vy = vx0, vy0
  i = 0
  TkAfter.new( 50, 42, proc{
    c.move('item', vx, vy)
    i += 1
    vy = vy0 + 0.7*i }).start
end

TkButton.new(nil, 'text'=>'go', 'command'=>proc{ throw( c ) }).pack

Tk.mainloop