The Brachistochrone: Why the Fastest Descent Is a Cycloid

Grant Sanderson talks with mathematician Steven Strogatz about the brachistochrone: the curve of fastest descent between two points under gravity. In the standard setup, the particle starts at rest and slides without friction in a uniform gravitational field.

The shortest path is not necessarily the fastest. A steeper initial descent gains speed sooner, but adds distance; the problem asks for the best balance.

Strogatz recounts Galileo’s interest in the question and Johann Bernoulli’s 1696 challenge. They tell a well-known story about Newton’s anonymous solution while acknowledging that part of the anecdote is legendary.

Bernoulli’s insight relates a sliding particle to light passing through layers in which speed changes. Fermat’s principle and Snell’s law provide a way to express the time-minimizing path.

Conservation of energy gives speed proportional to the square root of vertical drop. In the layered-light analogy, the ratio of the sine of the tangent’s angle to the vertical to that square root remains constant along the limiting path.

The resulting curve is a cycloid, traced by a point on the rim of a rolling circle. Sanderson then presents a geometric argument attributed to mathematician Mark Levi for why the cycloid has the required property.

The wheel’s contact point is the instantaneous center of rotation. Using perpendicular tangents and similar triangles, the construction relates the drop to the sine of the tangent angle and shows the required ratio is constant.

Sanderson ends with a further challenge: describe the sliding motion using the tangent angle as a function of time. For this idealized motion on a cycloid, the associated wheel angle changes at a constant rate, suggesting a different way to look for a proof.