Science Questions

Why do glasses, cups and plates have a ridge around the base?

Sun, 15th Jun 2008

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Adrian asked:

Why do glasses, cups and plates have a ridge around the base? Why not have a flat bottom?


If you imagine a plate with no ridge and there is some small piece of grit, food or dirt on the table the plate would rock and make it difficult to eat off it. However if you have a ridge it is very unlikely the grit is under the ridge itself and even if it is when you move the plate slightly the plate would fall off the grit becoming stable again.

It is also probably more difficult to manufacture a perfectly flat piece of pottery than a ridge that is the same height all the way around.


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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? adrian , Thu, 15th May 2008

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. lyner, Thu, 15th May 2008

:)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.

adrian , Fri, 16th May 2008

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.

that mad man, Fri, 16th May 2008

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. qazibasit, Sat, 14th Jun 2008

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. Bored chemist, Sat, 14th Jun 2008

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