Matter & Energy · Depth 3 · Introductory · 2 min read
Forces
A 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.
What is a force?
A force is a push or a pull on an object. Every force has a size (its magnitude) and a direction, which is why physicists treat forces as vectors: two forces can add up or cancel out, depending on which way they point.[1]
Some forces need contact, like a floor pushing up on your feet. Others act across empty space through a field. Gravity is the everyday example: Earth pulls on you without touching you.[1] Forces are measured in newtons (N). One newton is the force that gives a 1 kg mass an acceleration of 1 metre per second, every second.[1]
Forces and motion
What matters for motion is the net force, meaning all the forces on an object added together.[1] Newton’s laws say that with no unbalanced force, an object stays still or keeps moving at the same speed in a straight line. An unbalanced force makes it accelerate, and a heavier object needs more force to speed up by the same amount.[2]
Example: the four forces on an airplane
A plane in flight feels four forces:[3]
- Weight: gravity pulling it down, toward the centre of the Earth.
- Lift: from the wings, holding it up.
- Thrust: from the engines, pushing it forward.
- Drag: air resistance, holding it back.
When these forces balance, the plane cruises at a constant velocity. When they don’t, it accelerates in the direction of the largest force.[3]
Real-life examples
Standing still
Right now gravity pulls you down while the floor or chair pushes you up. They cancel out, so you don't move: forces can balance.[1]
An airliner at cruising altitude
When lift balances weight and thrust balances drag, a plane cruises at a constant velocity. Open the throttle and it speeds up.[3]
Pushing a heavy crate
Push gently and nothing happens: friction pushes back just as hard. Only when your push beats friction's limit does the crate start to move.[4]
Go deeper
5 of 6 topics below this one are written so far. The rest are uncharted: on the map, but not yet written. The Atlas is growing.
- Newton's Laws of MotionThree rules that explain how every object moves: inertia, force equals mass times acceleration, and every action has an equal and opposite reaction.
- GravityThe attraction between all masses. Why it weakens with distance, why the Moon 'falls' around Earth, and how it creates tides and keeps planets in orbit.2 branches
- FrictionThe force that resists surfaces sliding over each other. Why it exists, why starting is harder than keeping going, and how we use it or fight it.
Connected across the map
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.
- 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 meaning of force, net force and the newton unit are foundational physics, backed here by peer-reviewed university textbooks and NASA.
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 2 institutions: OpenStax, NASA.
Show all 4 sourcesHide the list
- 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
- National Aeronautics and Space Administration· Government agencyFour Forces on an AirplaneOpened and checked against this page on 28 Sept 2026
- ScholarlyOpenStax (Rice University)· Academic publisherCollege Physics 2e, 5.1 FrictionOpened and checked against this page on 28 Sept 2026 · License: CC BY-NC-SA 4.0