Has the law of gravity suddenly taken a vacation? Au contraire. Is the trickster in this video using sleight of hand? Nope. Can you use this trick to pick up women at cocktail parties? Possibly. Let's analyze just what's going on here.
There's a fairly substantial weight of water in the glass pushing directly down on the card. So what keeps the card (and water) from falling to the ground? The only possible answer is that there must be an upward force acting on the card, thus suspending it in place. But what is the nature of that force?
Let's recall a little fluid mechanics. Molecules are capable of exerting electrostatic forces on each other. In the case of like molecules these are called cohesive forces. For example water molecules exert cohesive forces on neighboring water molecules. Meanwhile unlike molecules (for example water molecules and playing card molecules) exert adhesive forces. In the video it appears that both adhesion and cohesion are in play. The adhesion between the water and the card (and the water and the glass) keeps everything "glued" together while the cohesion between water molecules prevents the water from "breaking" even when some fluid starts to leak out of the bottom of the glass. What's surprising is how strong these forces can be. I tried this trick using a completely full 550 milliliter bottle of water. That's around 1.2 pounds of water. The card held. However, make sure you use a glass with a relatively wide and flat rim or the forces won't be sufficient to impress your friends, neighbors and potential dates.
Is the vacuum in the top of the jar exerting the same force thought the liquid, like the pressure would be if >>>turned right side up<<<?
If so then the small opening between the card and the jars neck would be enough to resist the water flowing from the jar.
One way to test this is...
To make a small hole in the jars bottom and cover it with a finger. Then as the jar and card is rotated card down and stabilized,
Remove the finger from the hole, With predictable results. I think vacuum plays an equal or greater part in the trick.
You're absolutely right. When you turn the jar upside down and the water flows down onto the card, the space left behind has a lower pressure than the outside atmosphere. The difference in pressure means that there will be an upward force on the card and water which opposes the downward weight force (this is the same principle that causes "suction"). While adhesion and cohesion are sufficient to hold up the card when there is little or no water in the jar, for any significant weight of water the difference in pressure is necessary to make the trick work.
Thanks for this important clarification.