[adrian]

adrian  asked the Naked Scientists:

Hi Naked ones,

Why do almost all the glasses, cups, plates you see have that edge at the bottom, rather than a flat surface?

(Once I bought such a cup, with a flat bottom. It was a dangerous cup...).
 
Regards,

Adrian


What do you think?

[lyner]

With a flat bottom, they will slide around on a smooth table top if there is any water there - aquaplaning. The pressure under the ridge keeps good contact with the table - like the tread on tyres.

[adrian]

:)This was a simple one:)
I would add that having the ridge is more stable when the table is not flat, it has pieces of ..anything underneath the cup. I was saying that the cup that I had (with a flat base) was dangerous not only because of this, and because of the aquaplaning, but also because when you lift something with a flat base from a wet surface, you have a vacuum. So you apply more force to raise the cup, which gives suddenly.

[that mad man]

I cant say much about ceramics except I believe it helps in the firing process as Ceramics shrink when fired so it helps not to have the base completely in contact with the surface it stands on.

Most glass vessels these days are moulded and not hand blown. The edges are then rounded off with a flame and not ground down which makes thing easier and cheaper. Doing this leads to a rounded lip or edge.

Have a look a cheap wine glasses and you will notice a lip on the top rim and the bottom, now look at some hand blown crystal and you wont see any.

[qazibasit]

its to hold ur plate on the table and to prevent it from slipping while ur eating there is also another mechanism and it is that there is a vaccum created at the base of ur plate a low pressure vaccum and it basically holds ur plate on the surface.

[Bored chemist]

If you want a "flat" base, ie one that won't rock on a flat surface you have to grind the base of the thing flat. If the thing has a rim, you only need to grind that flat. Otherwise you need to grind the whole of the base flat.
The argumenst about friction don't tally with the idea of a coeficient of friction. The area in contact with the table doesn't matter. The friction force is that same anyway.
Very few tables or plates are anything like smooth enough to maintain any noticable vacuum(except, possibly when wet) for long enough to make a difference.