Everyday Physics: suction cups.

You 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.

 

A cross section of a suction cup attached to a wall

 So, how do these little suckers work? That’s the post for today. I have not explained any device recently, so I thought these would be a nice addition to the everyday physics series.

 

You will have noticed that the suction cups tend to be made of some deformable plastic substance. If you or I press them against a wall, the suction cups deform in shape. You will also notice that an airtight seal is formed by the edges of the suction cup. In particular a small pocket of air forms between the cup and the wall, but this is not in the natural shape of the suction cup.
Instead, the pocket is smaller than it should be. Also, the cup is a plastic substance that wants to restore it’s shape. It obeys Hook’s law, which I explained before. Sort of.

So what is the deal? 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. That is pretty cool for a device without batteries, don’t you think?

Now, here are a few extra things to consider for the proper behavior of suction cups.

 

 

1. Should you wet the edged before applying? Usually yes. This is because the water will help make the seal tight. Once it sets it does not matter. This is the ‘use spit’ instruction.
2. Make sure that you use it against a non-porous surface. Porous surfaces let air in (sometimes slowly), so if you use in in one of these surfaces it will eventually fall. Glass is very non-porous. Sandstone is usually very porous: suction cups don’t usually stick to these.
3. Bigger is better. Usually the pressure is proportional to the area, so bigger suction cups suck more.
4. To remove, it is easier if you peel it from the sides, rather than try to pull it away from the wall. The ‘peeling’ action lets air in, and then there the suction cup stops sucking.
5. Finally, it is sometimes fun to use suction cupped assorted objects and put them on your face for decoration. This can make an excellent halloween costume.

Also, just for fun, I added an interactive poll at the end to amuse you with whatever answers other people might leave behind.

 

 

Now, if I could only figure how to edit a typo in the poll….

About these ads