--- title: "Deformation & Fluid Pressure" description: "Explain elastic and inelastic deformation, thermal expansion and contraction, and how pressure in still fluids acts in all directions and increases with depth" canonical: https://lightmysky.com/learn/science/deformation-and-fluid-pressure-mt_fkwcCB5px7 source: https://lightmysky.com/learn/science/deformation-and-fluid-pressure-mt_fkwcCB5px7.md retrieved: 2026-10-19 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Deformation & Fluid Pressure Explain elastic and inelastic deformation, thermal expansion and contraction, and how pressure in still fluids acts in all directions and increases with depth Subject: Science · Area: Forces & Motion · Ages 12 to 13 Page: https://lightmysky.com/learn/science/deformation-and-fluid-pressure-mt_fkwcCB5px7 ## Ready when they can - Distinguishes elastic deformation (returns to shape) from inelastic/plastic deformation (permanently changed) - Explains how expansion gaps in some bridges and jointed railway tracks allow thermal expansion and contraction - Explains how pressure grows with depth in still water, compares extra pressure at fixed density, and compares equal-depth pressures when water density, gravity and surface pressure match - Gives an everyday example of thermal expansion causing a problem or being used usefully - Uses equal added pressure to calculate added forces on ideal, frictionless hydraulic pistons at the same height, ignoring piston weights - Finds total absolute pressure by adding air pressure of about 101 kPa to gauge water pressure ## Lesson: Bending, Expanding, and Pushing Back You have worked out how forces add up and cancel out. Push, pull, bend, or squeeze an object hard enough and you deform it: you change its shape. What happens once you stop pushing depends on how far you went. **Example.** Stretch a rubber band gently and let go. It returns to its original shape. That is elastic deformation: the shape change is reversed once the force is gone. **Example.** Now bend a paperclip a little. It springs back too, same as the rubber band. But bend it too far and it stays bent. Past a certain point the material cannot return to normal. That permanent change is called inelastic, or plastic, deformation. Most solids expand when warmed and contract when cooled. Their particles move farther apart on average as they warm. A warmed metal lid can expand and loosen. Some bridges and jointed railway tracks use gaps to allow expansion. If expansion is blocked, thermal stress can buckle metal. Other tracks use special engineering instead of gaps. *(drawing: Extra pressure in still water, in relative units. With constant density, deeper water gives taller bars.)* Pressure is perpendicular force per area. In still water, it acts in every direction and increases with depth. With constant density, extra pressure = density x gravity x depth. Doubling depth doubles this extra pressure. At fixed depth and surface pressure, the pool's shape does not matter. Higher pressure near a dam's bottom calls for more support. A submarine needs a hull strong enough for its diving depth. In an ideal enclosed liquid, an added pressure spreads equally (Pascal's principle). With frictionless pistons at the same height, added force / area is equal. A 100 N added push gives 1000 N on ten times the area. We ignore piston weights. Absolute pressure = air pressure + gauge pressure: 101 kPa of air plus 50 kPa of extra water pressure gives 151 kPa. **Recap.** Elastic changes reverse; plastic changes remain. Most materials expand when warmed. In still water, pressure acts in every direction and increases with depth. ## Practice 16 questions on this page, each with its working shown. ## Needs first - [Resultant Forces](https://lightmysky.com/learn/science/resultant-forces-mt_AiWlJfvC3O) ## Opens up - [Pressure in Liquids and the Atmosphere](https://lightmysky.com/learn/science/pressure-in-liquids-and-the-atmosphere-mt_eLGNkEJAZl)