On the physics of pressure: how the microscopic world interacts with us Premium

Discover the fascinating world of pressure in physics, from air pressure to blood pressure, explained by an IIT Kanpur professor

We again have a leopard visiting us on the IIT Kanpur campus. We already have porcupines, a million dogs, nilgais, snakes, and scorpions. Leopards are a new addition to this thriving ecosystem. It’s probably something about the wet monsoon air of Kanpur that beckons these creatures, as well as the students and faculty members. The courses are starting again now after the summer break, as is the grind. As students old and new gather, the academic pressure begins to rise.

Pressure is an interesting thing: we often don’t like it and try to avoid it, but it is indispensable. It’s everywhere and there is a bit of wonderful physics hiding in it.

Pressure is how the microscopic world interacts with us. We don’t see billions of atoms and molecules around us in the air but they hit us continuously all the time, creating what we call air pressure. When you are taking a flight, if there is a risk of the number of air molecules reducing around you, the flight host tells you “the pressure may fall, please use an oxygen mask”. If you want to cook something where you want these air molecules to be more energetic — to strike your vegetables better — you use a pressure cooker. In fact, before putting your vegetables in the cooker, you add water. These water molecules become hot and impact your carrots and potatoes, breaking their molecular bonds and cooking your food. You keep all these molecules locked in a strong metal vessel they can’t escape, increasing the pressure.

But what really is pressure?

Pressure and force are deeply related. You may have heard of Isaac Newton as just another of those long-haired physicists, but the basic unit of force is named in his honour: newton (N). A unit is a thing you use to measure quantities or other aggregates of some entity. For example, we measure distances in metres, weights in kilograms, and the amount of milk in litres. Likewise, force — such as when you push or pull a door — can be ‘counted’ in newtons. A fruit, like a small apple, weighs roughly 100 g and will exert a force of about 1 N on your hand. In fact, after a bout of intermittent fasting, if you weigh unreasonably low — like 50 kg — you can instead say you weigh 500 N.

Pressure is the average force spread over any area. Imagine a heavy book is resting on your palm. Now imagine you’re holding the same book up with just one finger. The weight of the book remains the same but it feels heavier because it rests on a tinier region.

A pascal (Pa) is a unit of pressure. One pascal is really a very small amount of pressure. For instance, the same apple on your hand exerts a pressure of about 500 Pa. All the air on top of our heads, right up to space, exerts a pressure of 100,000 Pa — like about 200 apples on your hand!