A student drops a small iron nail and a wooden ruler into a trough of water. The nail sinks to the bottom whereas the wooden ruler remains floating on the surface. The student then takes a thin iron sheet of the same mass as the nail, bends it carefully into the shape of a tiny boat and places it on water; this time the iron boat floats.
Answer the following questions based on this observation.
(i) Define density and state its SI unit.
(ii) Using the idea of density, explain clearly why the iron nail sinks while the wooden ruler floats.
(iii) Give a reason, in terms of average density, for the iron boat floating even though it is made of the same material as the nail.