--- title: "Momentum and Its Conservation" description: "Momentum is mass times velocity, and it points the way the object moves. With no outside force, the total momentum of a group of objects is the same after a collision or explosion as before." canonical: https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_ source: https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_.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`. # Momentum and Its Conservation Momentum is mass times velocity, and it points the way the object moves. With no outside force, the total momentum of a group of objects is the same after a collision or explosion as before. Subject: Science · Area: Forces & Motion · Ages 14 to 16 Page: https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_ ## Ready when they can - Calculates momentum in kg m/s and gives its direction - Works out an unknown velocity after two trolleys collide and stay joined - Applies the same reasoning to an explosion, where two objects start at rest and move apart ## Lesson: How motion is shared in crashes Momentum measures how much motion an object carries, which matters when you brake a bike or catch a ball. You find it by multiplying mass by velocity. Its unit is kilogram metres per second, written kg m/s. Because velocity has a direction, momentum points that way too. **Example.** A 2 kg trolley rolls at 3 m/s. Multiply 2 by 3 to get 6 kg m/s in the direction it rolls. A slow lorry can share the same momentum as a fast tennis ball when the numbers work out. Big momentum means hard to stop. When trolleys collide with no outside force pushing in, the total momentum stays the same, because the action-reaction pairs between them cancel. Add every momentum before, and the total after must match. If two trolleys stick together, one joined lump carries it all, so the same momentum shared by more mass means a slower speed. Explosions run the same rule backward from rest. The total starts at zero and stays at zero, so the parts move apart in opposite directions with momentums that cancel. Pick one direction as positive and keep the signs straight. **Tip.** Explosions run the same rule backward. Two trolleys start at rest with zero total momentum, then push apart in opposite directions. Pick one direction as positive and keep the signs straight, because opposite momentums cancel to zero. **Recap.** Mass times velocity gives momentum, and the total stays the same through crashes and explosions. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [What happens when things collide](https://lightmysky.com/learn/science/what-happens-when-things-collide-mt_1MLi55bPnt) - [Terminal Velocity](https://lightmysky.com/learn/science/terminal-velocity-mt_3eNYHV-m-c) - [Newton's Third Law](https://lightmysky.com/learn/science/newtons-third-law-mt_fbYbe3YSVj) - [Newton's Second Law with Numbers](https://lightmysky.com/learn/science/newtons-second-law-with-numbers-mt_Nn2MVJL7bO) ## Opens up - [Impulse and Crash Safety](https://lightmysky.com/learn/science/impulse-and-crash-safety-mt_BjLAFw2zjU) - [Elastic and Inelastic Collisions](https://lightmysky.com/learn/science/elastic-and-inelastic-collisions-mt_JMwUuh1YUq)