Kinematics: Describing Motion

This course opens the Science Subject's second arc — the physics of everyday, human-scale objects, starting with the simplest real question physics can ask: how do you precisely describe something moving? Kinematics answers that question with real, exact tools, and along the way corrects one of physics' own most persistent popular myths.

Position, Velocity & Acceleration: Real Definitions

Position describes where an object is; velocity describes how fast, and in what direction, that position is changing; acceleration describes how fast the velocity itself is changing. These three real quantities build directly on each other — velocity is the real rate of change of position, and acceleration is the real rate of change of velocity.

The Real SUVAT Equations

For motion with constant (uniform) acceleration in a straight line, five real equations — commonly called the SUVAT equations, after their variables: displacement (s), initial velocity (u), final velocity (v), acceleration (a), and time (t) — connect all the real motion involved:

• v = u + at
• s = ut + ½at²
• s = ½(u + v)t
• v² = u² + 2as
• s = vt − ½at²

Each real equation uses exactly four of the five variables — meaning any three known values are enough to solve for the other two.

Galileo and Falling Bodies: A Real, Corrected Myth

A popular story holds that Galileo dropped two different-mass objects from the Leaning Tower of Pisa to prove they fall at the same rate. Most historians consider this real event never actually happened as a physical demonstration — the story's sole source is Vincenzo Viviani, Galileo's own student and biographer, writing in 1654, decades after Galileo's death, and not published until 1717. Notably, Galileo himself left no real account of any such experiment. Historians generally treat it as a thought experiment described later, rather than a real, physically performed test.

An Even Earlier Real Experiment

Galileo wasn't even first: Flemish scientist Simon Stevin conducted a real, similar falling-body experiment in Delft in 1586 — years before Galileo's own work — demonstrating the same real underlying principle. These real findings, refined over time, eventually led to the equivalence principle: the real idea that gravitational acceleration and inertial acceleration are indistinguishable — later extended by Einstein into general relativity, a real connection this Science Subject's own sibling course, Electromagnetism & Relativity, returns to directly in its own closing chapter.

The Five Real SUVAT Equations, at a Glance

Hands-On Exercises