--- title: "Non-Newtonian Fluids | College Physics I" description: "Non-Newtonian fluids change viscosity when shear changes, so their flow in College Physics I differs from Newtonian fluids in pipes, blood, and sauces." canonical: "https://fiveable.me/intro-college-physics/key-terms/non-newtonian-fluids" type: "key-term" subject: "College Physics I – Introduction" unit: "Unit 12" --- # Non-Newtonian Fluids | College Physics I ## Definition Non-Newtonian fluids are fluids whose viscosity changes with shear stress or shear rate. In College Physics I, they show up when you compare real fluids like blood, ketchup, and paint to the simpler Newtonian model. ## What It Is In College Physics I, non-Newtonian fluids are fluids that do not keep a constant viscosity when the force on them changes. Instead of following a straight-line relationship between shear stress and shear rate, their resistance to flow can go up or down as they are stirred, squeezed, or pushed through a pipe. That is the big difference from Newtonian fluids. For a Newtonian fluid, like water under normal conditions, doubling the shear rate doubles the shear stress, so the viscosity stays the same. For a non-Newtonian fluid, the fluid can thin out, thicken up, or even act like it is not flowing at all until enough stress is applied. One common type is shear-thinning. The more you push or stir it, the easier it flows. Ketchup is the classic example, and blood also behaves this way in many situations. This is why some fluids seem stubborn in a bottle but move more easily when you shake or squeeze them. Another type is shear-thickening. Under faster or stronger shear, the fluid gets harder to move. A cornstarch-and-water mixture is a familiar demo because it can feel liquid at rest but resist suddenly when hit or pushed quickly. Some non-Newtonian fluids also have a yield stress, which means they need a minimum force before they start moving at all. Toothpaste is a good everyday example. It stays in the tube until you apply enough pressure, then it flows out. For physics, the point is not just memorizing examples. You look at how the fluid responds to force, then decide whether a constant-viscosity model works. In pipe flow problems, that choice changes how you think about resistance, flow rate, and whether a simple equation like Poiseuille’s law can be used without extra assumptions. ## Why It Matters Non-Newtonian fluids matter in College Physics I because the fluid model you choose changes the story you tell about motion. If you assume a fluid has constant viscosity when it really changes with shear, your predictions for flow rate, pressure drop, or resistance can be off. This shows up right next to topics like viscosity, laminar flow, and Poiseuille’s law. Those ideas are built around smooth flow and a steady relation between pressure difference and volume flow rate. A non-Newtonian fluid can break that simple pattern, especially if its viscosity depends on how fast layers are sliding past each other. The concept also helps explain real-life fluids that students already know. Blood is not just a textbook fluid, and materials like paint, shampoo, and polymer mixtures behave differently depending on how they are handled. That makes non-Newtonian flow a useful bridge between the equations in class and the messy behavior of real materials. In lab or problem-solving, you may be asked to classify a fluid from a description, predict what happens when the shear rate changes, or explain why a familiar equation is not a perfect fit. Knowing this term helps you spot when the simple Newtonian model works and when the fluid itself is changing the rules. ## Connections ### Viscosity Viscosity is the measure of a fluid’s resistance to flow, and non-Newtonian fluids are defined by the fact that this resistance is not constant. In a Newtonian fluid, viscosity stays the same at different shear rates. In a non-Newtonian fluid, you may have to describe how the viscosity changes instead of quoting one fixed value. ### Laminar Flow Laminar flow is smooth, layered motion, and it is the flow pattern most closely tied to the simple fluid models in this topic. Non-Newtonian fluids can still flow laminar, but their changing viscosity can affect how easily they keep that layered motion. If the fluid thickens under stress, the flow pattern can become harder to predict. ### Poiseuille's Law Poiseuille’s law describes flow rate for a Newtonian fluid moving through a pipe under laminar conditions. Non-Newtonian fluids are the cases where you have to pause before using that formula, because the law assumes constant viscosity. In a problem, spotting a non-Newtonian fluid tells you the standard equation may need modification or may not apply cleanly. ### [Reynolds number](/intro-college-physics/key-terms/reynolds-number) Reynolds number is used to judge whether flow is likely laminar or turbulent, based on fluid speed, size, and viscosity. For non-Newtonian fluids, the viscosity part is trickier because it may change with shear. That means Reynolds number can still be useful, but you have to be careful about what viscosity value you use. ## On the AP Exam A quiz item might give you a fluid description and ask whether it is Newtonian or non-Newtonian, or it may ask which behavior matches shear-thinning, shear-thickening, or yield stress. In a problem set, you might compare how two fluids respond when the shear rate increases, or explain why a pipe-flow equation fits water better than ketchup. Lab questions often ask you to describe the shape of a stress-versus-shear-rate graph or interpret a demo such as cornstarch suspension, blood analogs, or paint flow. The main move is to connect the observed change in flow to the fluid’s changing viscosity, not just to memorize an example. ## Non-Newtonian Fluids vs Newtonian Fluids Newtonian fluids keep the same viscosity at a given temperature, so shear stress changes linearly with shear rate. Non-Newtonian fluids do not. If a question says the fluid gets thinner, thicker, or needs a threshold force to start moving, you are no longer in Newtonian territory. ## Key Takeaways - Non-Newtonian fluids are fluids whose viscosity changes when the applied shear changes. - A Newtonian fluid has a constant viscosity, but a non-Newtonian fluid can shear-thin, shear-thicken, or require a yield stress to start flowing. - In College Physics I, the term shows up when you compare real fluids with the simpler assumptions used in pipe-flow equations. - Examples like ketchup, toothpaste, blood, and cornstarch mixtures make the behavior easier to picture. - When you see this term, ask how the fluid responds to force, not just whether it is a liquid or gas. ## FAQs ### What is non-Newtonian fluids in College Physics I? Non-Newtonian fluids are fluids whose viscosity changes when the shear on them changes. In College Physics I, they are the main counterexample to the simpler Newtonian model where viscosity stays constant. You usually meet them when comparing real materials like blood or ketchup to ideal fluid equations. ### What is the difference between non-Newtonian fluids and Newtonian fluids? Newtonian fluids keep a constant viscosity, so the relationship between shear stress and shear rate is linear. Non-Newtonian fluids do not follow that pattern, and their viscosity can drop, rise, or wait until a threshold force is reached. That difference matters when you are using flow equations in pipe problems. ### Is ketchup a non-Newtonian fluid? Yes, ketchup is a classic non-Newtonian fluid, and it often behaves as shear-thinning. That means it flows more easily when you shake, squeeze, or stir it harder. This is why it can seem stuck in the bottle at first and then come out faster. ### Why do non-Newtonian fluids matter in pipe flow problems? Pipe flow formulas in this unit often assume a Newtonian fluid with constant viscosity. If the fluid is non-Newtonian, the pressure drop and flow rate can change in ways that the simple model does not capture well. That is why identifying the fluid type is part of solving the problem correctly. ## Related Study Guides - [12.4 Viscosity and Laminar Flow; Poiseuille’s Law](/intro-college-physics/unit-12/4-viscosity-laminar-flow;-poiseuilles-law/study-guide/OZpBfSDENdUfACdU) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/intro-college-physics/key-terms/non-newtonian-fluids#resource","name":"Non-Newtonian Fluids | College Physics I","url":"https://fiveable.me/intro-college-physics/key-terms/non-newtonian-fluids","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/intro-college-physics/key-terms/non-newtonian-fluids#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:15.477Z","isPartOf":{"@type":"Collection","name":"College Physics I – Introduction Key Terms","url":"https://fiveable.me/intro-college-physics/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/intro-college-physics/key-terms/non-newtonian-fluids#term","name":"Non-Newtonian Fluids","description":"Non-Newtonian fluids are fluids whose viscosity changes with shear stress or shear rate. 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