Nothing You Do Pulls the Drink Up a Straw

The atmosphere does.

Drawing in your breath does not pull the liquid. What it does is remove air from inside the straw, lowering the pressure there. The air pressing down on the surface of the drink in the glass is now the greater pressure, so it pushes the liquid up the straw and into your mouth.

The proof is easy: make a pinhole in the side of the straw above the liquid and no amount of effort will bring the drink up, because air now leaks in and the pressure inside the straw can never fall.

This page covers the third part of the ICSE Class 8 Physics chapter on force and pressure: how pressure depends on area, how it grows with depth in a liquid, how large atmospheric pressure is, and the devices that put it to work.

Because the pressure of a liquid increases with depth:

$$P=h\rho g$$

where $h$ is the depth below the free surface, $\rho$ is the liquid's density and $g$ is taken as $10\text{N/kg}$.

Worked example. In water of density $1000{\text{kg/m}}^3$, at a depth of $5\text{m}$:

$$P=5\times 1000\times 10=50000\text{Pa}=50\text{kPa}$$

At $20\text{m}$:

$$P=20\times 1000\times 10=200000\text{Pa}=200\text{kPa}$$

Four times the depth, four times the pressure. A dam wall therefore has to resist far greater sideways push near its base than near the top, so it is built thick at the bottom and tapered towards the top — matching the load instead of wasting material.

Three properties of liquid pressure.

- It increases with depth, as the formula shows.
- It acts equally in all directions at a given depth — sideways and upwards too, not only downwards. This is why a hole in the side of a tank squirts water outwards, and why the upthrust of the previous chapter pushes up.
- It depends on the density of the liquid. At the same depth, a denser liquid exerts more pressure — mercury far more than water.

What it does not depend on. Not the shape of the container, and not the area of its base. Connect vessels of wildly different shapes and the liquid settles to the same level in all of them, because the level is set by depth alone. A narrow tall tube and a wide tall tank have identical pressure at the same depth, even though the tank holds far more water.

This is why a village water tank is placed on a tall tower: the pressure at a tap depends on the height of the water above it, not the size of the tank. It is also why a tap on the ground floor runs harder than one on an upper floor of the same building.

The air above us has weight, and that weight presses on everything below. At sea level this atmospheric pressure is about

$$1.01\times 10^5\text{Pa}$$

Put into force. On a palm of area about $0.01{\text{m}}^2$:

$$F=PA=1.01\times 10^5\times 0.01\approx 1000\text{N}$$

That is the weight of a body of mass around $100\text{kg}$, pressing on one hand.

So why are we unharmed? Because the pressure of the fluids inside the body — blood and other body fluids — pushes outwards with an equal pressure. The two balance, exactly as the book and table balanced in the first part of this chapter. The atmosphere is not gentle; it is simply matched.

When the match is broken, the effect is dramatic. Boil a little water in a thin metal can, cap it and let it cool: the steam inside condenses, the inside pressure drops, and the can is crushed by the atmosphere it had been resisting all along.

Atmospheric pressure falls with altitude, because there is less air above you. At high altitude the reduced pressure is why breathing is harder, why a sealed packet of snacks swells on a mountain road, and why aircraft cabins are pressurised. It is also why water boils below $100{}^{\circ }\text{C}$ up a mountain, as the previous chapter described.

And it acts in all directions, not just downwards — which is what makes the devices in the next section work at any orientation.

All of them work the same way: create a region of lower pressure, and let the atmosphere push.

A drinking straw. Sucking removes air from the straw, so the pressure inside falls below atmospheric. The air pressing on the drink's surface pushes liquid up the straw.

A syringe. Pulling the piston back increases the space inside and lowers the pressure there, so atmospheric pressure drives the liquid in through the needle. Pushing the piston does the reverse and expels it.

A rubber sucker. Press it against a smooth wall and most of the air underneath is squeezed out. Atmospheric pressure then holds it firmly in place. For a sucker of area $0.002{\text{m}}^2$:

$$F=1.01\times 10^5\times 0.002\approx 202\text{N}$$

enough to hold a heavy towel. The wall must be smooth — on a rough or porous surface air seeps back underneath, the pressure difference vanishes, and the sucker drops off.

A dropper or ink filler. Squeeze the bulb to expel air, dip the tip in the liquid, then release: the pressure inside is now lower and the atmosphere pushes the liquid up into the tube.

A siphon. A tube full of liquid is used to empty a tank into a lower vessel without lifting it out. Once the tube is filled and the far end hangs below the liquid level in the tank, flow continues on its own — the longer column on the outlet side outweighs the shorter one inside the tank, and atmospheric pressure on the tank's surface keeps pushing liquid up into the tube to replace what leaves. Raise the outlet above the tank's level and the flow stops at once, because the pressure difference that drove it is gone.

A water pump with a plunger lifts water the same way, and a vacuum cleaner lowers the pressure inside its body so the atmosphere drives air — and dust — in through the nozzle.