The two example cited are actually still in wide use: The cog is in analogue clock, engine and other mechanical device that have mechanical parts, and the magnifying glass can be bought from your local store (and without the handle it is pretty much what any other optical device is on a very basic level).
The floppy disk on the other hand ... well sure we still use magnetic medium to save data on a daily basis without even thinking about it, but there might be only a couple generation that would have knowingly handled such a device. It makes sense for them. But for our parents and children? It is quite an abstraction that 'saving' needs to have a symbol on its own pretty much like the 'on/off' switch does: http://en.wikipedia.org/wiki/Power_symbol.
The floppy disk on the other hand ... well sure we still use magnetic medium to save data on a daily basis without even thinking about it, but there might be only a couple generation that would have knowingly handled such a device. It makes sense for them. But for our parents and children? It is quite an abstraction that 'saving' needs to have a symbol on its own pretty much like the 'on/off' switch does: http://en.wikipedia.org/wiki/Power_symbol.