Question

why does a suction cup stick to a smooth surface like glass?

Answer 1

because this suction cup creates a vaccum like enviroment

Answer 2

ou might have seen them in your shower, kitchen refrigerator or walls, or even you might have seen them hanging from the glass panel in the rear of a car you are following. They are suction cups, the product that really sucks. Erm, maybe that is not the best description of these objects, since they are very cool and useful and they don’t suck. Well, they do, but in the sense of suction, not in the other colloquial sense.

The plastic substance wants to restore it’s shape of course. This process increases the size of the air pocket, but no air can get in. Therefore an amount of gas occupies more volume and it has less pressure inside than on the outside. The air pressure from outside pushes the suction cap stronger from the outside than from the inside, so there is a net force pushing the suction cup against the wall. That force is balanced by the wall. The fact that the air has less pressure inside than out is the ‘sucking’ action.

In the end, the outside air is pressing the cup against the wall. Under these conditions friction can work (proportionally to the pushing force), and it will hold the suction cup in place.