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# A Cycloid is a Tautochrone

#### A Short Proof

In 1659 Christiaan Huygens answered a question that came to him as he watched a swinging chandelier in a church: What curve has the property that a bead sliding along it under uniform gravity and with no friction will oscillate with a period independent of the amplitude?

The answer turned out to be the cycloid generated by a circle, as illustrated in Figure 1. Several solutions to this problem have been found; Abel’s is particularly remarkable [1].

Figure 1. The contact point C is an instantaneous center of rotation of the rigid wheel, and thus vr.

Presented here is a very short geometrical proof of the tautochronous property of the cycloid. It is based on the fact that $$\bf{v} \perp \bf{r}$$, as explained in Figure 1.1

To prove that the cycloid is a tautochrone, it suffices to show that the arclength distance $$s$$ from the bottom of the cycloid behaves as a harmonic oscillator:

$a = −ks,$

where $$a = \ddot{s}$$, for some constant $$k$$. (This idea, which I had learned from Henk Broer, is attributed to Lagrange.) Because $$a = 0$$ when $$s = 0$$, we just need to verify that

$\tag{1} da = −k ds.$

But $$da = d(g ~\mathrm{cos}~ \theta) = −g ~\mathrm{sin}~ \theta~ d\theta$$. And from Figure 2 we have $$ds = D sin \theta d \theta$$. Comparing these expressions for $$da$$ and $$ds$$ proves (1) with $$k = g/D$$. QED

Figure 2. Proof that a = –ks, using the fact that PC'PC.

1 Incidentally, building on this fact, the line of velocity of every point on a rolling wheel (in the ground reference frame) passes through the topmost point of the wheel. A pebble stuck to the tire always aims straight at, or straight away from, the topmost point of the wheel!

References
[1] M. Levi, Classical Mechanics with Calculus of Variations and Optimal Control, AMS, Providence, Rhode Island, 2014.

Mark Levi ([email protected]) is a professor of mathematics at the Pennsylvania State University.