* That comes from classical mechanics, a field of study that you’re about to invent right here in this very footnote! Classical mechanics is concerned with describing how objects respond to forces that act upon them, and the three laws we’re about to describe are its foundation. Before they were formalized in 1686 CE by Isaac Newton, people had a less-perfect understanding of why and how things moved, and had to make do with worse theories like “Rocks love the ground and smoke loves the sky, so that’s how come smoke rises and rocks don’t” (Aristotle, 300 BCE). The three laws of motion are: (1) Objects at rest remain at rest, and objects in motion remain in motion, unless acted on by a force. (2) The rate of change of an object’s momentum is directly proportional to the force applied, and is in the direction of the applied force. (3) For every action, there is an equal and opposite reaction: as you push a box forward, the box also pushes you back. Keep in mind: we call these laws, but they’re really an approximation that happens to work great at human scales. When you get very small (quantum scales of less than 10–9m), very fast (2.99×108 m/s, close to speed of light), or very heavy (black holes), they tend to fall apart, and here Einstein’s theories of general and special relativity describe motion with more accuracy than classical Newtonian physics can. But that’s nothing you need to concern yourself about! While gravitational time dilation caused by an accelerating inertial reference frame moving through highly curved areas of space-time is a fascinating topic—and one that’s also extremely useful when constructing FC3000™ rental-market time machines—unless you happened to bring your copy of Good Grief: You Say Space and Time but Two Aspects of a Single Continuum Called Space-Time, and Furthermore That the Speed of Light in a Vacuum Is the Same for All Observers of That Light Regardless of the Motion of the Light Source? Well, Here Are 1001 Educational Comics About General and Special Relativity That Explore Those Ideas Further back in time with you, we wouldn’t worry about it just yet.