Air pressure’s power to halt horses and carry cardboard, without pulverizing people
At first, my kids referred to what they were seeing as a “trick,” like it was part of a magic show. It was actually a well-known, kitchen-science demonstration about air pressure, but it was new and wondrous to them in a way that elicited furrowed brows and excited disbelief. I was simply holding a cup full of water upside-down, with a piece of cardboard “floating” or “sticking” underneath it. My daughter was surprised at how much the cup of water weighed, because, somehow, neither it nor the cardboard was falling all over the kitchen. “It’s like a suction cup,” she finally ventured, although it would be a bit longer before she could explain why that idea made sense.
“Wait, would this work on the Moon? With no air?”
Even though my demonstration may not seem like magic anymore, it’s good to keep in mind that for huge swaths of recorded history, most people wouldn’t have been able to really explain why my cardboard wasn’t falling away from the inverted cup. The idea that atmospheric air pressure was even “a thing” wasn’t easily tested until it could be experimentally removed by comparing normal conditions to a vacuum. Since Aristotle, people accepted the idea that “nature abhors a vacuum” to mean that vacuums just couldn’t really exist. So when German scientist Otto von Guericke visited Holy Roman Emperor Ferdinand III in 1654 for a demonstration of his new invention, nobody in the audience had any way to predict what they were about to see.
Von Guericke arrived with a 20-inch, hollow, bisected sphere and his new vacuum pump. The two halves of the sphere were placed so they touched, but their point of contact was smooth, with just a bit of grease between them. If the intersection was an equator, the “poles” of the sphere had a valve to pump out air, plus hooks that would be used later. The pump was hooked up to pump all the air out of the sphere, at which point teams of horses were hooked up on either side to try and pull the two halves apart. They couldn’t do it.
“So, there was nothing inside, but…”
Not only had von Guericke demonstrated making a vacuum inside his copper sphere, he also showcased the power of the Earth’s atmosphere. The air on the outside of the sphere was pushing the two pieces together so hard it outmatched the strength of the pulling horses. If it had been supported vertically, the bottom half of the sphere would have stayed in place even with over 4,000 pounds hung underneath it- around the weight of an adult rhinoceros. The demonstration was understandably a sensation, and the sphere was even named a Magdeburg sphere, after von Guericke’s home town. And of course, it’s basically the same concept that was holding my cardboard against my cup.
“Carrying a car?”
All that said, it was still hard for my kids to accept that the invisible air that we basically ignore all day could somehow exert so much force. We don’t see it happening, and we don’t even feel it, which seems weird, considering it’s like we’re supporting a Honda Civic all day. We don’t feel the rhino-toting power of the air all the time because unlike the Magdeburg sphere, we normally have air molecules pushing us from all directions, and they balance each other out to a degree, unless it’s windy of course.
Secondly, organisms on Earth have had millions of years to adapt to living with the air pressure here. The 2,204 pounds of pressure you can ignore coming down on your head is balanced and accommodated by the internal pressure of the various fluids that make up your body. Your blood, muscles and lungs are all full of substances pushing outwards, and they’re tuned to expect external pressure from the air pushing back. Without the right amount of pressure pushing on our bodies, things get ugly pretty fast.
At this point, my second grader seemed like the pieces were fitting together. We talked about popping ears on airplanes, the feeling of water pressure under a pool, and watching bags of chips puff up at high altitudes where there’s less air pressure. Hopefully she stays content with the cardboard demonstration, although if not, at least further tests won’t require access to a team of horses.
Source: Storm in a Teacup: The Physics of Everyday Life by Helen Czerski, Transworld Publishers Limited, 2016, p.15