Every fact in the three friction lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the lessons simplify: f = μ·N is an approximate, experimentally found model, and the coefficients of friction (from OpenStax) are given to only one or two digits; real values depend on how clean, dry and worn the surfaces are, so your classroom results may differ. The cause of friction is still a research topic: bumps on the surfaces and attraction between surface molecules (adhesion) both play a part. The youngest lesson uses the 'tiny bumps' picture recommended in the Victorian science continuum. The statement that rolling friction is 100 to 1,000 times smaller than sliding friction (Britannica) is a general range, not an exact figure.
All sources
[1] University Physics Volume 1, 6.2 Friction
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Friction is a force that opposes relative motion between systems in contact; static friction (no sliding) f_s ≤ μ_s·N and kinetic friction f_k = μ_k·N; static friction is usually greater than kinetic; friction does not depend on the area of contact, because only high spots touch; surfaces are rough close up and part of friction is due to adhesion between surface molecules; on a horizontal surface the normal force equals the weight; an object slides down a slope at constant velocity when μ_k = tan θ. Approximate coefficients (static/kinetic): rubber on dry concrete 1.0/0.7, rubber on wet concrete 0.5–0.7/0.3–0.5, wood on wood 0.5/0.3, waxed wood on wet snow 0.14/0.1, steel on steel dry 0.6/0.3, oiled 0.05/0.03, ice on ice 0.1/0.03, human joints 0.016/0.015. Worked example: a 100 kg crate (weight 980 N) with μ_s = 0.45 needs more than 440 N to start; with μ_k = 0.30 it needs 290 N to keep moving at constant speed. The model is approximate and the values are given to one or two digits.
Friction is the force that resists the sliding or rolling of one solid object over another; friction such as the traction needed to walk without slipping can be beneficial; the force to start motion is greater than the force to continue it; friction is nearly independent of the area of contact; coefficients of sliding friction are generally 100 to 1,000 times greater than those of rolling friction; surfaces are microscopically irregular and adhesion is a major cause of friction between metals.
Victorian Department of Education (Australia) · Reliability: High
Type: Government teacher guidance
Friction is thought to be caused by interactions between tiny bumps on surfaces as they rub; friction acts on a surface in the opposite direction to its motion; friction can slow things down and stop stationary things from moving; friction can be reduced by lubricants; not all friction is 'bad'; pupils often do not see friction as a force.
Monash University and Victorian Department of Education (Science Continuum F–10) · Reliability: High
Type: University and government teacher guidance
Friction can slow things down and stop stationary things from moving; all surfaces have tiny bumps (sandpaper or a sandal sole is a good model); suggests exploring why shoes for running, rock climbing and skating are designed so differently; suggests tilting a board with a book on it, and comparing a slide with water and with soapy water, as predict-observe-explain tasks.
[5] College Physics 2e, 7.5 Nonconservative Forces
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Friction converts kinetic energy into other forms, including thermal energy; a car brought to a stop by friction loses kinetic energy, which is dissipated as thermal energy; the work done by friction over a distance d is W = −f·d.
[6] Physics, 4.2 Newton's First Law of Motion: Inertia
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
When the air of an air hockey table is turned off, the puck slides only a short distance before friction slows it to a stop; without friction a sliding box would keep sliding at constant velocity.
NASA Glenn Research Center, Beginner's Guide to Aeronautics · Reliability: High
Type: Space agency education page
Drag opposes motion through the air and can be thought of as aerodynamic friction; part of it is skin friction between air molecules and the surface; it depends on the velocity difference between object and air and, for form drag, on the shape.