Fill a glass half with water, place a card over the top of the glass, completely covering it. Hold the card in place while you invert the glass, release the card and it stays in place.
The book that I first saw this trick in gave this as the explanation:
“The weight of the water pushing down on the card is not equal to the force of air pressure pushing up agains the card, and so the card stays in place”
OK, but what about the air that’s in the glass? Surely that air is exerting pressure downwards on the water, which in turn transmits the force down on to the card? Wouldn’t this have the effect of cancelling out the force of the air pressure from underneath?
Is the explanation I read all those years ago wrong, or is there something I’ve missed?
The card covering the cup is not perfectly rigid, so it starts to bulge out when you invert the cup. This causes the trapped air to expand. And if you expand a limited amount of gas its pressure decreases, while the outside air pressure remains constant. At some point the water plus air pressure inside the cup will balance the outside air pressure. Hopefully before the card bends too far and air sneaks in through the gap.
Actually, it’s not really air pressure pushing UP against the card, it’s the fact that the glass is airtight.
The airbubble trapped above the water is “fixed”–that is, it has no way to expand or contract its volume. In order for the water to push out the card, the air bubble has to expand. It can’t.
The card has basically stabilized the system by causing the water’s shape to be flat on top and below, so that its “push” downward is flatly even, and the air bubble’s “hold” force is also flatly even.
Perhaps someone else can explain it more scientifically…
“The airbubble trapped above the water is “fixed”–that is, it has no way to expand or contract its volume. In order for the water to push out the card, the air bubble has to expand. It can’t.”
By that rationale, if you half fill a bottle of water, put your finger over the top, invert the bottle and release your finger, the water will stay there, held by the “hold” force of the air bubble in the bottle. This does not appear to be the case in my experience.
SCR4 wrote:
“The card covering the cup is not perfectly rigid, so it starts to bulge out when you invert the cup. This causes the trapped air to expand. And if you expand a limited amount of gas its pressure decreases, while the outside air pressure remains constant”
Hmmm…atmospheric pressure is 14.7 PSI, say a surface area of card of 6 square inches and a weight of water of 0.5 lbs- so we’d need a pressure drop inside the glass of a little under 0.1 PSI- roughly 1%. I could believe that. Anyone care to shoot the calculations down in flames?
Looks good to me. To cause that pressure drop, the volume of the air (not the whole glass) needs to expand by 1%. The less air there is in the glass, the easier this is.
It’s a 1% change in colume of the trapped air, not a 1% curvature of the card. If you manage to do it with no air at all, the card won’t have to curve at all.
Surface Tension…Not air pressure
Try this cool trick. Take a canning jar and cut a ring of wire mesh (screen) make the ring big enough so that you can hold it in place with the jar ring. Fill the jar with water, put your card on top, turn upside down and remove the card. The water should stay up as long as you don’t tilt the jar.
And on a more juvenile note when I was in high school we did this to rude waitresses–we did the glass trick, set the whole thing down on the table, then slipped the card out.
Hammos, you’re right: If you hold a half-filled bottle upside down, release your finger, the water runs out. But notice how the water runs out. Bubbles have to go up to replace the water coming out, in order to maintain the pressure previously mentioned.
The “bottom” of the water–that part at the bottle’s opening–isn’t stable enough in its surface tension to prevent water from falling out and air bubbles from rushing in to replace it. But add the card, and you’ve stabilized the system. No air bubbles can rush up, enabling the existing air pocket’s “hold pressure” to keep the water from falling down.