Every fact in the three forces and motion 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: the youngest lesson treats every force as a simple push or pull, which is the description the Victorian science continuum recommends at that level. The value of g is rounded to 9.8 m/s² on Earth (OpenStax gives 9.80 m/s²) and 1.67 m/s² on the Moon. In the trolley experiment, friction and air resistance are never exactly zero, so measured accelerations are usually a little lower than F/m predicts. Newton's laws are an excellent description at everyday speeds; at speeds close to that of light or at atomic scales other physics is needed, which these lessons do not cover.
All sources
[1] Physics, 4.1 Force
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
A force pushes or pulls an object; forces are vectors with magnitude and direction; a free-body diagram represents the object as a single point; the net external force is the total combined external force; equal and opposite forces are balanced and add to a net force of zero; force applied to an object changes its motion; Newton's laws describe how objects speed up, slow down, stay in motion and interact.
[2] Physics, 4.2 Newton's First Law of Motion: Inertia
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
A body at rest tends to remain at rest; a body in motion remains in motion at a constant velocity unless acted on by a net external force; inertia is the tendency to keep the same motion and is proportional to mass; when the air of an air hockey table is turned off, the puck slides only a short distance before friction stops it; without friction a sliding box would continue at constant velocity.
Gives F_net = m·a, where only the net external force affects the acceleration; the newton (N) is the force that gives a 1 kg mass an acceleration of 1 m/s² (1 N = 1 kg·m/s²); acceleration is inversely proportional to mass; weight W = m·g with g = 9.80 m/s² on Earth and 1.67 m/s² on the Moon; worked example: a net force of 51 N on a 240 kg lawn mower gives 0.21 m/s².
States Newton's three laws: objects at rest stay at rest and objects in motion stay in motion in a straight line unless acted upon by an unbalanced force; force equals mass times acceleration; for every action there is an opposite and equal reaction. A ball held in the hand has balanced forces (gravity pulls down, the hand pushes up); let it go and the forces become unbalanced; a resting rocket needs an unbalanced force to move; the rocket pushes on the gas and the gas pushes on the rocket; a skateboard rolls the opposite way when you jump off; a cannon is pushed backwards when it fires.
Any force, such as friction or gravity, that makes a body slow down or speed up is an unbalanced force; an unbalanced force gives a change of speed (acceleration or deceleration); inertia is proportional to a body's mass; with force in newtons, mass is in kilograms and acceleration in m/s².
Monash University and Victorian Department of Education (Science Continuum F–10) · Reliability: High
Type: University and government teacher guidance
At primary level it is appropriate to describe a force as a push or a pull; a force is exerted on one object by another and is not something an object 'has'; gives the pull of a magnet on a paper clip as an example; describes the common misconception that a moving object contains a force that gets used up.
NASA Glenn Research Center, Beginner's Guide to Aeronautics · Reliability: High
Type: Space agency education page
Drag is the aerodynamic force that opposes motion through the air and acts opposite to the motion; it can be thought of as aerodynamic friction; it comes from the difference in velocity between the object and the air; part of it (form drag) depends on the object's shape.