Yes João, your assumption of 'relative' is right. 'Relative' in the sense that it walks ON the elastic band at 1cm/sec. Relative to any of the thumbs it is of course a different story: Plot a graph and you will see that the ant at first removes itself very slowly from its starting point and, eventually, approaches the right thumb very fast. The ant speed therefore is continually changing relative to the thumb(s). Thanks for your elegant solution!