* Hint: Smaller bottles with a narrow neck are easier to handle. It's as simple as that. 1). For students who already have the concept of air pressure, it's often worthwhile to let the class brainstorm about why the water stays in the glass before leading them through an explanation. Use a cylindrical glass instead of a tapered glass to make the calculation a little easier. ... is a little easier than a bottle with a narrow mouth and a wide base. A glass that is tapered, with the base smaller than the mouth as in Fig. If the flyscreen is hard to cut, stop. Try using a foam picnic plate instead of an index card. Does the water soak through the index card too quickly and make a mess? This air pressure is pushing up on the card from below, while the water is pushing down on the card from above. Put an index card over the mouth of the glass and press the palm of your hand on the index card, pressing the card against the rim of the glass and depressing it slightly into the glass in the center (this part is very important). While your hand is on the index card over the mouth of the glass, invert the glass and slowly take your hand away. Have them derive an expression for the distance the water must fall in order to balance forces. Make sure the glass is perfectly rigid. The trick is an upside-down cup with water in it. File attachments, if any, are at the bottom of the page. The water molecules don’t stick together particularly strongly, but it’s enough to keep air out and water in. It's an easy winnable bet, once you know the trick. Practice this trick a few times. Turn a glass of water upside down without letting the water fall out. Water tends to stick to itself, and this property is known as surface tension. Note for geeks: In the case of the tapered glass, it might be tempting to think that even if the air pressure were the same on top and bottom, the force pushing down on the water from above is smaller than the force pushing up from below because the area is smaller above the water than below. To make cutting easier, keep the scissor blades apart. Once the card sags enough so that these three forces balance, everything will stay put. Why doesn't the water fall out of the glass with the index card? Use the first centimetre or so of the blades that are closest to the handles. However, any bottle with a small mouth will also work just fine. Practice this trick a few times. This is explained very well. The air can’t stretch the surface of the water from a tiny flyscreen square to a huge bubble. Give the gift of science this holiday season with a discount subscription to Double Helix! In the upside-down glass, it helps prevent the first water drop from separating from the rest of the water volume. In a narrow straw, there isn't enough room in the opening for both a drop of water to fall out and a bubble of air to flow in at the same time. You now have water on the floor. 2, is a little easier than a bottle with a narrow mouth and a wide base. The secret to this trick involves some basic lessons in air pressure. Learn how your comment data is processed. posted on 13 Aug 2013 by guy Remember to press into the glass a little bit before you turn it over. For much smaller openings, surface tension is enough to stabilize the surface, and we actually don't need the index card. Hurry as it’s a strictly limited offer! Clicking on an image displays the full-size image, if available. While the raised sections of the cap can make it harder to remove, they're also where the magic lies. Water molecules have a strong attractive "cohesive" force between them due to the fact that each water molecule can make four hydrogen bonds with other water molecules. : This is a pretty simple trick that can be done with items you can find at home. Can you pull out a dollar bill from under an upside down beer bottle, without knocking the bottle over? However, this argument fails to take into account the force from the sides of the glass. How to Put Water Into an Upside-down Cup! Cover the opening with your thumb and turn the bottle upside down. You can see how weak the force between water molecules is by tipping the bottle sideways. As a result, the air pressure goes down a tiny bit according to Boyle's Law. But when Benedict squeezes the bottle, the pressure is too strong and the cellophane falls off, letting the water out. Then you can shock your friends and family next time you pour them a glass of water. Try using a lighter more flexible material across the mouth of the glass. If the glass is tapered, the sides of the glass exert a force that has a small downward component, and this component exactly makes up for the reduced area directly above the water. The reason for this is that in the case of the bottle, the card has to sag by a bigger amount in order to generate the necessary volume (and pressure) change. When you want to stop the water flowing out, keep tipping until the bottle is completely upside down. The left sidebar shows related lessons and curricula that link to this lesson, or that the author has explicitly recommended, as well as modified copies of the lesson if any. As a result, the pressure difference required to keep the water in the glass is less than would be needed if there were no cohesive force. Try pouring some water out of the bottle and into the sink.