Matter & Energy · Depth 2 · Introductory · 1 min read
Mechanics
The branch of physics that explains how and why things move: forces, motion, momentum and energy, from falling objects to orbiting planets.
What is it?
Mechanics is the part of physics that describes motion and what causes it. Its core ideas (space, time, mass, force, momentum, torque and angular momentum) were first worked out to solve one of the most famous problems in science: predicting how the planets move.[1]
At the heart of mechanics are forces. A force is a push or a pull, and it always has both a size and a direction.[2] Newton’s laws of motion connect forces to motion. An object keeps doing what it is doing unless an unbalanced force acts on it. A force makes a mass accelerate (F = m × a). And every force comes with an equal and opposite partner.[3]
Key ideas
- Forces change motion. Pushes, pulls, gravity and friction are all forces, and the total (net) force decides how an object’s motion changes.[2]
- Two ways to solve a problem. You can follow the forces step by step, or use conservation laws, such as conservation of energy and momentum, that hold no matter how complicated the details.[1]
- Fluids follow the same rules. Fluid mechanics applies these ideas to liquids and gases, from air pressure in the weather to why ships float.[4]
Where to go next
Follow Forces to see how pushes and pulls add up, or Fluid Mechanics to see how the same laws let an airplane fly.
Real-life examples
Predicting the planets
Mechanics grew out of one of the most famous problems in science: explaining and predicting how the planets move.[1]
A jet engine
A jet engine pushes hot gas backwards, and the gas pushes the engine forwards: Newton's third law at work.[3]
A floating ship
A steel ship floats because water pushes up on it (buoyancy), the same mechanics that makes a hot-air balloon rise.[4]
Go deeper
14 of 15 topics below this one are written so far. The rest are uncharted: on the map, but not yet written. The Atlas is growing.
- ForcesA force is a push or a pull with a size and a direction. Forces start, stop and steer every motion, including an airplane's flight.3 branches
- Fluid MechanicsHow liquids and gases behave when they sit still and when they flow: pressure, buoyancy and the physics behind weather, pipes and flight.3 branches
- Energy & WorkWork is energy passed on by a force. How energy of motion and stored energy trade places, why the total never changes, and what power measures.
- MomentumMomentum is mass times velocity. Why it is conserved in collisions, how an impulse changes it, and how airbags and rockets use it.
Learn more
Short descriptions are our own summaries. The resources belong to, and are run by, their publishers.
- University Physics Volume 1 (free textbook) ↗
by OpenStax
A free, peer-reviewed university textbook covering mechanics, including forces, Newton’s laws and fluid mechanics.
- 8.01SC Classical Mechanics ↗
by MIT OCW
MIT’s complete first-year mechanics course with videos, notes and problem sets, free to use.
- Beginner’s Guide to Aeronautics ↗
by NASA
NASA Glenn’s free, illustrated guide to how airplanes and rockets fly, from the four forces to engines and wind tunnels.
Evidence & sources
Supported by extensive evidence and broad scientific consensus.
Why this level? The ideas on this page are foundational, long-established physics, and every point is backed by NASA, MIT or a peer-reviewed university textbook.
This is a Knowledge Atlas editorial classification of the sources we could find, not a certificate of truth. How we evaluate knowledge
Sources
Based on 4 sources from 3 institutions: MIT OCW, OpenStax, NASA.
Show all 4 sourcesHide the list
- MIT OpenCourseWare· University8.01SC Classical Mechanics (course description)Opened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0
- ScholarlyOpenStax (Rice University)· Academic publisherUniversity Physics Volume 1, 5.1 ForcesOpened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0
- National Aeronautics and Space Administration· Government agencyNewton's Laws of MotionOpened and checked against this page on 28 Sept 2026
- ScholarlyOpenStax (Rice University)· Academic publisherUniversity Physics Volume 1, Chapter 14: Fluid Mechanics (Introduction)Opened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0