---
title: "Pressure in Liquids and the Atmosphere"
description: "Pressure is force spread over area. In a still fluid, pressure increases with depth and fluid density. Pressure differences explain upthrust; average object density helps predict floating, and air pre"
canonical: https://lightmysky.com/learn/science/pressure-in-liquids-and-the-atmosphere-mt_eLGNkEJAZl
source: https://lightmysky.com/learn/science/pressure-in-liquids-and-the-atmosphere-mt_eLGNkEJAZl.md
retrieved: 2026-11-12
---

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# Pressure in Liquids and the Atmosphere

Pressure is force spread over area. In a still fluid, pressure increases with depth and fluid density. Pressure differences explain upthrust; average object density helps predict floating, and air pressure falls with altitude.

Subject: Science · Area: Forces & Motion · Ages 15 to 16
Page: https://lightmysky.com/learn/science/pressure-in-liquids-and-the-atmosphere-mt_eLGNkEJAZl

## Ready when they can

- Calculates pressure from force and area, and pressure at depth from depth, density and gravitational field strength
- Explains upthrust as the pressure difference between the bottom and top of a submerged object
- Predicts whether an object floats by comparing its average density with the liquid's, and explains why air pressure falls with altitude

## Lesson: Force spread over area

Pressure spreads a force over an area. Divide force by area to get pressure in pascals, where one pascal is one newton per square metre. The same weight on a stiletto heel presses far harder than on a snowshoe. Pins and knives work by shrinking the area.

Dive deeper and the pressure climbs. The extra pressure from the liquid equals depth times density times gravitational field strength, so doubling the depth doubles it. Denser liquids pile it on faster than light ones. That is why a dam wall is thickest at its base.

**Example.** Lena holds a block under water. Water pushes on every face, but the bottom face sits deeper and feels more pressure than the top, leaving a net upward push called upthrust. To predict floating she compares densities: an object whose average density is less than the liquid's floats, and a denser one sinks.

Car tires use gauge pressure, measured against the air around us. A reading above that air is positive, and a reading below it, as inside a partial vacuum, is negative. Absolute pressure adds the atmosphere on top. Air itself thins with height, since less air piles above you, so mountain air presses less.

**Recap.** Pressure is force over area, and depth piles it higher.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Deformation & Fluid Pressure](https://lightmysky.com/learn/science/deformation-and-fluid-pressure-mt_fkwcCB5px7)
- [Density and How to Measure It](https://lightmysky.com/learn/science/density-and-how-to-measure-it-mt_HrKI1-UzfI)
- [Floating, Sinking and Upthrust](https://lightmysky.com/learn/science/floating-sinking-and-upthrust-mt_TJI95Ot-qQ)

## Opens up

- [Gas Volumes and the Ideal Gas Equation](https://lightmysky.com/learn/science/gas-volumes-and-the-ideal-gas-equation-mt_I1fz_6wjuD)
- [Continuity and Bernoulli's Equation for Ideal Flow](https://lightmysky.com/learn/science/continuity-and-bernoullis-equation-for-ideal-flow-mt_pKxlVYTy0Y)
- [Hydrostatic Equilibrium and the Structure of a Star](https://lightmysky.com/learn/science/hydrostatic-equilibrium-and-the-structure-of-a-star-mt_RKeMW8iX56)