Pressure and Fluids

Why ships float, straws work, and the air around you is heavier than it feels.

What pressure actually is

Pressure is one of those words people use loosely. In physics, it’s exact:

Pressure = force spread over an area. $P = \frac{F}{A}$

Measured in pascals (Pa). One pascal is one newton per square meter — a tiny amount.

The why-it-matters example

Step on a Lego with bare feet. Agony. Why? Because all your weight (let’s say 700 N) is pressing on the tiny pointy area of one stud (maybe 0.000002 m²).

$P = \frac{700}{0.000002} = 350{,}000{,}000 \text{ Pa}$

350 million pascals into the bottom of your foot. That’s why it hurts so much more than standing on a flat floor with the same body weight.

Same force. Tiny area. Massive pressure.

This is also why:

• Knives cut (thin edge → small area → huge pressure).
• Snowshoes work (big area → less pressure → don’t sink).
• High heels dent floors but elephant feet don’t (yes, really).

Fluids: liquids and gases together

In physics, “fluid” means anything that flows: liquid or gas. Water is a fluid. Air is a fluid. Honey is a fluid (just a slow one). The rules for liquids and gases are mostly the same, which is convenient.

You live at the bottom of an air ocean

The air above you is pulled down by gravity, just like everything else. That column of air pressing down on you right now is called atmospheric pressure, and it’s about 101,000 Pa (101 kPa).

That means roughly 10 newtons of force on every square centimeter of your body. Your hand has about 100 N of air pressing on each side.

Why don’t you feel crushed? Because the same pressure pushes on you from every direction — and you have fluids inside pushing back at the same pressure. It all balances. The moment something unbalances it (like sucking on a straw, or going up in a plane), you notice immediately.

How a straw really works

This is the moment-of-truth example. You don’t suck liquid up a straw. The atmosphere pushes it up.

1. You lower the pressure inside the straw

   By sucking, you remove some air from the top of the straw. Lower pressure inside than outside.

2. Atmosphere pushes the drink up

   The full atmospheric pressure is still pushing down on the rest of the drink. Liquid takes the easy path — up the low-pressure straw.

3. You drink it

   Congratulations, you used the atmosphere as a tool.

This is why straws have a maximum height. If your straw was over 10 meters tall, even a perfect vacuum at the top couldn’t lift water that high — the atmosphere just isn’t strong enough.

Pressure in a liquid

Dive into a pool. Your ears hurt the deeper you go. Why?

Because the deeper you are, the more water is above you pressing down. The rule:

$P = \rho g h$

• $\rho$ (rho) = density of the liquid
• $g$ = gravity
• $h$ = depth

Deeper = more pressure. Linearly. Every 10 meters of water adds about 1 atmosphere of pressure. Submarines have to be built like tanks because the deep ocean crushes things.

Pressure depends only on depth, not on the shape of the container. A tall thin tube of water and a wide swimming pool of the same depth have the same pressure at the bottom. This freaks people out, but it’s true — and it’s how hydraulic systems work.

Why ships float: Archimedes’ principle

A 100,000-ton steel ship floats. A 1-gram steel marble sinks. What gives?

Anything in a fluid is pushed up by a force equal to the weight of the fluid it pushes out of the way.

This upward push is called buoyancy.

• The marble pushes aside a tiny amount of water (weight: tiny). Buoyant force is way less than the marble’s weight → it sinks.
• The ship, hollow and huge, pushes aside a lot of water (weight: 100,000 tons). Buoyant force = the ship’s weight → it floats.

It’s not about the material. It’s about how much water you shove out of the way.

A quick test

Why does ice float on water? Because frozen water is less dense than liquid water. Same mass takes up more space. So a piece of ice pushes aside more than its own weight in liquid water. Buoyancy wins. (This is also weirdly rare — most things are denser when solid. Water is special, and life on Earth depends on it.)

Pascal’s principle: how hydraulics multiply force

Pressure applied to a fluid in a closed container spreads equally everywhere.

This is the trick that lets a person lift a car with one hand. A car jack has two pistons connected by oil:

• Small piston: small area. You push with a small force.
• Big piston: huge area. Same pressure × huge area = huge force.

You trade distance for force. You have to pump the small piston many times to move the big one a little. But the energy is conserved — you’re just spreading your work out.

This is how:

• Car brakes work (foot pushes a small piston, brake calipers squeeze the wheel hard)
• Excavators lift tons of dirt
• Dentist chairs go up and down

Mechanical engineers love hydraulics. They turn small inputs into huge outputs.

Next: Heat and Temperature

What “hot” really means at the molecular level.