--- title: "Modelling Motion in a Straight Line" description: "Set up a motion problem before calculating anything: choose which direction counts as positive, treat the object as a particle, and keep displacement, velocity and acceleration apart from distance, sp" canonical: https://lightmysky.com/learn/mathematics/modelling-motion-in-a-straight-line-mt_no78_6w1rS source: https://lightmysky.com/learn/mathematics/modelling-motion-in-a-straight-line-mt_no78_6w1rS.md retrieved: 2026-09-12 --- > **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`. # Modelling Motion in a Straight Line Set up a motion problem before calculating anything: choose which direction counts as positive, treat the object as a particle, and keep displacement, velocity and acceleration apart from distance, speed and everyday words like slowing down. Subject: Mathematics · Area: Mechanics · Ages 16 to 17 Page: https://lightmysky.com/learn/mathematics/modelling-motion-in-a-straight-line-mt_no78_6w1rS ## Ready when they can - Give both the displacement and the distance travelled for an out-and-back journey - Say what a negative velocity and a negative acceleration each mean once a direction is chosen - State the assumptions hidden in the words particle, smooth and light ## Lesson: Setting up motion before calculating Turn the journey into numbers before calculating. Displacement runs straight from start to finish with direction attached. Distance is the full path length. Velocity pairs with displacement the way speed pairs with distance. Pick a positive direction first and stick to it. **Example.** A runner goes 5 m forward and 5 m back. Finish minus start is zero, so displacement is 0 m. The path covered is 10 m of distance. Average velocity is zero while average speed is positive. Keep the pair you were asked for. Signs carry the story. Negative velocity means moving against the chosen positive direction. Negative acceleration means velocity is decreasing. Slowing down means velocity and acceleration point opposite ways. No single sign means slow on its own. **Tip.** Models confess what they ignore. A particle has no size or spin. Smooth means no friction. Light means no mass worth counting. State these assumptions, since they bound how far the answer can be trusted. **Recap.** Choose a direction, split displacement from distance, read the signs, and state the assumptions. ## Practice 14 questions on this page, each with its working shown. ## Needs first - [Compound Units](https://lightmysky.com/learn/mathematics/compound-units-mt_kLQOzZYrd5) - [Vectors: Notation, Addition and Scalar Multiples](https://lightmysky.com/learn/mathematics/vectors-notation-addition-and-scalar-multiples-mt_NkqmrL7pLe) - [Choosing a Model and Naming What It Ignores](https://lightmysky.com/learn/mathematics/choosing-a-model-and-naming-what-it-ignores-mt_UtK9aauJ6e) - [Calculating with Bounds in Compound Measures](https://lightmysky.com/learn/mathematics/calculating-with-bounds-in-compound-measures-mt_zfMz7x8dlf) ## Opens up - [Vertical Motion Under Gravity](https://lightmysky.com/learn/mathematics/vertical-motion-under-gravity-mt_IgNj9bn14O) - [Motion Graphs: Gradients and Areas](https://lightmysky.com/learn/mathematics/motion-graphs-gradients-and-areas-mt_KhJeCMYBSL) - [Newton's Second Law: Force, Mass and Acceleration](https://lightmysky.com/learn/mathematics/newtons-second-law-force-mass-and-acceleration-mt_SKO85AcKdb)