Where’s the pressure difference in this trick?

At the opening of the glass. The water inside the glass weighs a pound at best, and, depending on the size of the opening of the glass, the air pressure is exerting 15-30 pounds upward on the bottom of the card. Guess who wins? Tip, when you get good at this experiment, try doing it over a friend’s head!

Materials: a glass, and an index card large enough to completely cover the mouth of the glass.

### 7 Responses to “Magic Water Glass Trick”

1. Not all experiments have a downloadable worksheet – I think there’s over 700 now that do, and we’re still creating new ones for you. If there’s one you’d like a worksheet for, here are the basic steps we go through when we build them.

First, we ask ourselves – what is this experiment really about? What’s the main goal? What is the one concept that we are learning here? Then we write that down.

Next, we look at what kind of experiment the video is demonstrating for that concept, and figure out a question that the experiment is answering. For the Magic Water Glass trick, it might be about air pressure, something like “Does it matter how much water is in the cup? Can the cup be half full, all the way full, nearly empty for this to work? Which water level works best?” Or: “Does warm water hold the card in place better than cold? And how warm exactly?” or “Does the shape of the cup affect how well the card stays in place?”

Do you see how we’re just getting curious about the experiment itself, and just asking questions?

Now, we pick ONE question to ask and make a data table.

And that’s it! Do you think you can give it a try? Ultimately you’ll want to be able to go through this process yourself. The worksheet downloads are really “training wheels”, and although parents really love them, it’s important to understand what they are about and how to use them and what they are really for.

In the meantime, we’re still working on another 800, and I’ll put this one on my list to do. Enjoy!

2. normafleming77 says:

aurora is there a worksheet for this experiment
Gordon fleming
thanks

3. Adhesion may be a part of what’s holding the card to the cup in the situation you described. This PDF download from Harvard might also explain this phenomenon. The section titled “Note to Teacher” mentions Boyle’s Law, which describes the inverse relationship between a contained gas’s pressure and volume. Even when there is only a small amount of water in the cup to make the seal between the cup and card wet, a little bit of that moisture starts to absorb in the card and leaves more volume in the cup available to the air inside. Since the amount of air in the cup stays constant, the gas must spread out to fill the newly available volume, thereby decreasing the air pressure inside. Higher pressure pushes, and the air pressure is higher outside the cup than inside, so the card stays sealed to the cup as long as it remains rigid enough to hold the seal.

4. L.W. says:

How do I know it’s not adhesion holding the card on the inverted cup. I notice that the card stays in place as long as there’s moisture at the “seal,” even if no water is in the cup (in which case pressure above and below the card are equal, so gravity wins if the card is dry).

5. It can be confusing, as people often short-cut the words “pounds-per-square-inch” (psi) with the word “pounds”, and they two aren’t the same at all! Here’s what’s going on: the standard pressure in our atmosphere is 14.7 psi. The atmosphere is pushing with this constant pressure on all things, including the underside of the index card. Here’s how you figure it out:

To covert this pressure to force (pounds), simply multiply the pressure by the area. F = P * A. First, find the area:

1. If your cup has a diameter of 2 inches, use your high school geometry formula (Area = pi * (d2) / 4) to get an area of 3.14 * (2 in)2 / 4 = 3.14 square inches.

2. Now multiply this area (3.14 sq. in) by your pressure (14.7 psi) to get 46 pounds. That’s the amount of force pushing up on the cup. (Sounds like a lot, doesn’t it? You’re just used to living in a sea of air that we don’t really notice it much…)

Inside the cup, however, is water. There’s probably 0.5 pounds of water in your cup (a pint of water is about a pound), and this force is pushing down. So you have 0.5 pounds of weight pushing down against 46 pounds pushing up. Who wins?

There’s another thing happening that’s more subtle, though. There’s a slight vacuum being pulled inside the cup when you flip it, which also helps keep the card in place. Can you figure out how this happens?

You can go further with this experiment by using a fine mesh screen (metal, cheesecloth, etc.) in place of the index card. If you use a metal screen, it’s going to be heavier than the card, so you’ll need to tape it into place or use the ring of screw-type mason jar to hold it into place (remove the circular metal insert first). When you flip it over, the water doesn’t leak through the holes at all! This is especially convincing if you fill the glass first by pouring water through the screen (which is covering the mouth of the glass) and then flip it over – it looks like the mesh is a one-way valve for water!

(Hint: This has more to do with surface tension than air pressure. We’ll cover more about cohesion and surface tension in our Bubblology lesson in Chemistry, but basically the water stays in the glass (or jar) because the water sticks together to form a thin membrane between the holes. If you lightly tap the bottom of the screen, you’ll see small bubbles appear!)

6. Tara Forsburg says:

I am having a hard time wrapping my head around the “pounds” part. Why is it 1 pound of water vs. 15-30 pounds pressure. I understand the psi thing (sorta), but I am having a hard time grasping the complete thought. I’m sure an easy explanation from you, Aurora, will fix me right up! BTW…we are just digging in and taking the experiments in order. We are really having a great time! Love all the videos!

7. Rani John says:

Very neat! Bravo!