Wow Your Friends: Cool (Science) Party Tricks
It is enjoyable to wonder about how things work in the world of science. Today let’s look at science from a different perspective: how it can make you the life of the party!
Here are two science tricks you can easily master and perform for your friends—and impress them by explaining the hows and whys. The first is pretty cool and the second is really cool.
Trick #1: A Tornado of Liquid
Your party task here is to fill up a punch bowl. Open the cap on a two-liter bottle of soda, turn it upside down, and glug, glug, glug, the bottle is emptied into the bowl in about 20 seconds. Not particularly interesting and not particularly cool.
But try this approach. After you turn the bottle upside down to pour, give it a quick, vigorous, circular swirl. The liquid in the bottle will begin to circulate, a tornado-like vortex will form, and—wow!—the soda will drain out in only six or seven seconds.
What’s going on? The glug, glug, glug of draining liquid alerts you to the fact that air is entering the bottle as liquid leaves. If that weren’t the case there would be nothing—no air, no liquid, no anything—to fill the space at the top (inverted end) of the bottle. The force of the air outside the bottle pushes up on the liquid with about 10 pounds of force. The liquid only weighs about four pounds. Without anything at the top of the bottle pushing down, 10 pounds would be more than enough to prevent the liquid from running out. Air must enter the bottle in order for liquid to come out.
Let’s take a step back. Air or “atmospheric” pressure—the same pressure you hear about in the daily weather forecast—is the weight of all of the air in the atmosphere, all the way up to space, pressing on a fixed amount of area down here near the surface of the Earth. The weight of all that air is about 15 pounds on every square inch of the Earth, or 15 psi.
Atmosphere pressure at sea level is sufficient to keep a column of water about 30 feet tall from draining out of a sealed tube. Your two-liter bottle isn’t anywhere near 30 feet high, so it must be “unsealed” in order for liquid to come out. Air needs to get into the bottle.
The faster air goes in to the bottle, the faster liquid comes out. The glug, glug, glug method is slow. The tornado method, which effectively opens up a pipeline for air to rush into the bottle, is fast. Fast, and cool!
Trick #2: Automatic Can Crushing
Your second party trick also relies on air pressure. You decide to crush an empty can against your forehead, a la John Belushi in the movie Animal House. Or maybe you just put it on the floor and stomp on it. Again, not particularly interesting and not particularly cool.
But try this. Put a few tablespoons of water in the can and place it on your stove over modest heat. Once the water begins to simmer, turn the can upside down and immerse it completely into a shallow pan of water. The can is instantly crushed. And not in any way you’ve seen before: the sides of the can get sucked in!
(Warning: use a potholder or tongs to hold the can as you turn it over. The can will be hot!)
What’s going on? First, atmospheric pressure presses on everything—in this case, the outside surface of the can. At around 15 psi, that amounts to almost 400 pounds squeezing in on the surface of the can. The reason cans don’t usually collapse is that what’s inside—either liquid or air—is pushing out, and the inward and outward forces balance.
The small bit of steam from the boiling water disrupts this balance by pushing most of the air out. When you turn the can upside down in a pan of water, the steam quickly turns back into water, which takes up way less space. But because the opening of the can is now under water, no air can get back into the can to fill the empty space. The partial vacuum in the can offers no outward pressure. Feeling almost no resistance, the inward force due to air pressure outside crushes the can. Almost instantly, and very cool!
Questions to ponder:
The can trick starts by putting just a few tablespoons of water into an empty can. How much water do you expect will remain in the can after you crush it using the power of air pressure? (Come up with an answer before you try the trick!)
The surface area of an average person is around 2,500 square inches. The force due to air pressure is therefore nearly 20 tons. Why don’t we all just get squished into pancakes?