How do suction cups work to hold things up?
In: Physics
Air is really light, but there is a lot of air above us in the atmosphere. The weight of all this air creates a force that pushes down on us. When you push a suction cup into a surface, there used to be air inside, but now there isn’t as much, creating a low pressure and thus low force region. The force exerted by the air inside is much less than the force exerted by the air around us, so the air pushes on the cup and holds it in place.
Get your hands wet and then press your palms together at right angles, really hard and squish the air out from between them. You have a temporary suction cup. Pull your hands apart directly, with no twisting. You can feel how much harder it is. The air pressure between your hands is much lower than the pressure outside, so your hands are being pushed together much harder than they are apart. Suction cups don’t work in outer space, for that reason. There’s no air pressure outside them. We often get them wet to make a seal under them so the air doesn’t immediately leak back in and ruin the effect.
The atmosphere is held down by gravity. Because gravity exerts a downward force on every atom of the atmosphere, the atoms down by the ground are under a lot of weight from the atoms above. This gives our atmosphere air pressure.
When there is a difference in pressure, the universe will try to equalize the difference. This causes high-pressure air to move to lower-pressure areas. If the air can’t move though, a static force is generated as the pressure tries, but fails, to move the air. We can take advantage of this force by placing a membrane (the suction cup) between the area of high and low pressure.
By pushing the suction cup against a flat surface, you squeeze most of the air out, leaving an area of low pressure between the surface and your suction cup. Since this suction cup exists in earth’s atmosphere, the surrounding air has a higher pressure. The higher pressure wants to move to the lower pressure area, but it can’t, because the suction cup creates a seal. The air never stops trying though, as long as the difference in pressure exists, the air will try to move through the suction cup. This attempt to move generates a force on the suction cup. That force presses the suction cup into the surface.
TLDR: Suction cups redirect gravity using the atmosphere.
They form a vacuum and since there is nothing to replace the air, it pulls itself to the wall or whatever you stuck it to.
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