How come when you pull a cup out of a full sink upside down it hugs the water a little bit


Like, I’m not sure how to explain it other than I used to play with this a lot but never understood it

In: Physics

If the rim of the glass is still underwater air cannot get it. No air in the water can’t get out.

When you pull the cup out of the water with the opening under the water, the normal behavior of the water is to flow until it is level. This means the water within the cup will want to have a level which is equal to that outside the cup. But by pulling the cup upward you are opening a void near the base of the cup which cannot fill with air, the base being solid material (as it must to keep water in when upright).

Because that void cannot fill with air it starts to pull a partial vacuum against atmospheric pressure, forcing some amount of water into the cup and raising the water level compared to outside. The force required to raise this water is extra force required to lift the cup, making it feel as if the cup is heavier or it “hugs the water” (an interesting way to describe it).

Pressure. Basically, nature doesn’t like having a vacuum, which is a truly empty space, with no air or water or anything in it. As you pull the cup up from the water, if the water didn’t come up with it, you would have a vacuum forming inside the cup. Air can’t get in, because the rim of the cup is underwater, so if the water left, nothing would be there. So, the water stays in the cup until the rim of the cup reaches the surface. Now, air can flow up into the cup as the water falls down, so the water is able to fall.