Newton's Laws of Motion
Isaac Newton published the three laws of motion in his Principia Mathematica (1687). They describe how forces change the velocity of a body. Together with the law of universal gravitation, they explain every orbit in the simulator.
First law: inertia
A body remains at rest, or moves in a straight line at constant velocity, unless acted upon by a net external force. In orbit, no engine is required to keep a satellite moving; what curves its path is gravity, not a lack of inertia. Without gravity the satellite would fly off tangent to its orbit in a straight line.
Second law: F = ma
The net force on a body equals the time rate of change of its momentum. For constant mass this reduces to F = m·a, meaning acceleration is proportional to force and inversely proportional to mass. In the simulator, the acceleration vector at every instant points from the spacecraft toward the central body, with a magnitude set by Newton's gravity law.
Third law: action and reaction
When body A exerts a force on body B, body B exerts an equal-magnitude, opposite-direction force on A. A rocket accelerates by expelling mass: the engine pushes propellant out the nozzle, and the propellant pushes the vehicle forward. The same principle couples a satellite to its primary — both bodies orbit their common centre of mass, although for an Earth-satellite system the Earth's motion is negligible.