Atmospheric Pressure and Force on Surfaces (PLA 8)

Ben Lear

Alright, welcome back to The Honors Element! I'm Ben Lear, and today we’re talking about something that’s, honestly, everywhere: pressure. Physical pressure, like what you feel when you press your thumb into, let’s say, a tennis ball versus the tip of a pen.

Morgan Vincent

Hey everyone! Morgan here. And yeah, pressure’s one of those things we just kinda live with, but rarely stop to think about scientifically. It gets interesting when you start to think through some of the everyday examples. If you’re standing in sneakers, your weight spreads over that larger sole area. But swap in high heels, oh boy, suddenly all that force is focused onto those tiny points. Way more pressure on that part of the floor, right?

Ben Lear

Exactly! Same force, smaller area, higher pressure. And, actually, you see this idea show up in all kinds of ways. I mean, if you’ve ever tried hosing down your driveway, you probably know that putting your finger over the nozzle speeds up the water and it feels like it’s blasting. That’s because you’re narrowing the area, so the pressure increases. Classic application.

Morgan Vincent

The official definition though, in chemistry, is pressure equals force divided by area or P equals F over A. The SI unit for force is Newton per square meter. In chemistry we will continue to refer to this as a Pascal, abbreviated Pa. One Pascal is actually a really tiny amount, impractically small for most real situations.

Ben Lear

Yeah, totally, nobody’s checking the tire pressure in pascals at the gas station! Instead you’ve got all these other units people use, with the most common being atmospheres, which is the average pressure at sea level. 1 atm is defined as 101,325 Pascals, or just over 100 kilopascals if you really wanna round it off.

Morgan Vincent

And then, there’s millimeters of mercury, mmHg, which is super common with barometers. This is also sometimes called “torr” in honor of Torricelli, that 17th century guy who first measured atmospheric pressure with a mercury column. One atmosphere is 760 mmHg, or 760 torr.

Ben Lear

You might spot p s i too, pounds per square inch, especially in the US for tires and sports equipment. One atmosphere is about 14.7 psi. So, we end up with some pretty handy conversions here: 1 atm equals 101.325 kilopascals, 760 torr, 760 millimeters of mercury, and roughly 14.7 psi.

Morgan Vincent

We’ll refer back to these units a lot in class and in your homework, especially when we get to calculating real-world pressures and forces, but for now, just remember the core idea: pressure’s all about the force, and it totally depends on the area you spread that force across.

Ben Lear

Right, and now that we’ve got the basics down, let’s look at where this pressure actually comes from in our environment—what is atmospheric pressure, and how do we measure it?

Morgan Vincent

Atmospheric pressure. Let’s imagine it. We all live at the bottom of an “ocean” of air, even though it’s invisible and we don’t feel it crushing us. Well, most of the time, anyway! But what’s actually happening is, all those gas molecules in the air are zipping around and constantly banging into everything, including us. That’s atmospheric, also called barometric pressure in action.

Ben Lear

And the pressure we feel depends on both how many gas particles there are above us, and how heavy the air column is. The deeper you are, the more air above you, the higher the pressure. That’s why atmospheric pressure is highest at sea level and gets lower as you climb a mountain, like Denver or, ya know, Everest, where the column of air is thinner and lighter.

Morgan Vincent

Exactly. At sea level, the pressure’s about 1 atmosphere. But let’s say you’re on top of Mount Everest, and the pressure there? It’s only about 0.3 atmospheres. That drop in pressure explains why people get altitude sickness and why, fun fact, you can’t really boil water at 100°C on Everest. Lower pressure means lower boiling point for water. Cooking pasta takes a whole lot longer!

Ben Lear

So how do we measure this atmospheric pressure? That’s where barometers come in. The classic mercury barometer, invented by Torricelli, works by flipping a tube filled with mercury upside-down into a mercury dish. The atmospheric pressure pushes on the mercury outside, balancing the weight of the mercury column inside. The height of that mercury column is a direct readout of atmospheric pressure, about 760 millimeters at sea level.

Morgan Vincent

And if you don’t want to use mercury, for all sorts of safety and environmental reasons, there’s the aneroid barometer. That one works with a little evacuated metal capsule and a spring; when air pressure changes, the spring compresses or expands, and the pointer moves. Super handy, especially in weather forecasting.

Ben Lear

By the way, if you ever wonder why your ears pop when you go up a tall building or ride in a plane, you’re feeling the pressure change. Quick drops mean your inner ear needs to catch up to the new outside pressure. And, just like we said, those barometric swings are actually tracked in weather stations, which helps predict storms and changes in wind strength.

Morgan Vincent

There’s this neat connection back to the chemistry: when the pressure drops, like during a storm, gas molecules are hitting things less frequently and with less force, so the barometer drops. If you’re ever stuck without a fancy barometer, sometimes you can spot these shifts by how hard it is to drink through a straw or get water to boil, especially at high altitudes.

Ben Lear

And it’s all about particles, kinetic energy, and how these very tiny collisions add up to the big, very real effect of atmospheric pressure.

Morgan Vincent

Speaking of the practical side, let’s bring it back down to the numbers. We will go a lot more in depth about barometers next time. Now how do we connect all this to actual calculations? Let’s figure out how much total force the atmosphere exerts on ordinary objects, using that pressure equation and some real examples.

Ben Lear

So we said earlier: pressure is force per unit area, P equals F divided by A. Rearranged, if you know the pressure and the area, you can find the total force: F equals P times A. Pretty straightforward, but let’s work through an example that really drives home just how much force the atmosphere is putting on, well, everything.

Morgan Vincent

Let’s say you’ve got a box, its top surface is 2 feet by 3 feet. So the area is 2 times 3, which is 6 square feet, but we should convert to our SI units of square meters. So, our area is 0.56 m².

Ben Lear

For sure. That would be about 0.56 meters squared.

Morgan Vincent

Thanks! Now, the atmospheric pressure at sea level is about 101,325 Pascals. That’s 101,325 newtons per square meter. Now, just multiply pressure by area to get the total force: F comes out to about 56,742 Newtons. That’s huge! 12,756 pounds! For context, that’s like a mid-sized car sitting on top of the box, just from the air, and we don’t even notice.

Ben Lear

Wild, right? It’s a ton. Actually, like six tons, but the air pushes evenly in all directions, so things don’t collapse under the weight.

Morgan Vincent

It all comes back to how pressure, area, and force are related. Smaller area, same pressure, bigger force per square. They feel different.

Ben Lear

Alright, well that’s it for this episode of The Honors Element. Morgan, always fun chatting with you, but I really like talking about pressure since we can’t see it but we definitely feel it!

Morgan Vincent

Thanks, everyone, for listening, and practice those pressure and force calculations. You’ll want ‘em for the next lecture. Take care, Ben!

Ben Lear

See you all next time. Same place, more chemistry, probably just as much pressure. Bye, Morgan! Bye everyone!