ROTATING ROD SPIRAL

 Curve studied by Johannes Bernoulli in 1742 (Opera, T IV, p. 248). Homemade name ; other names : spiral of the hyperbolic cosine, spiral of the hyperbolic sine.

 Polar equation: .

The rotating rod spiral is the trajectory of a massive ring sliding without friction on a horizontal rod in rotation around a centre when the ring goes to infinity. See on this website a derivation of the polar equation above.

 Case b =  a: the equation can be written . Case b = – a: the equation can be written . Case b = – 2a.

Note that the curve is similar to the curve in the first case if ab > 0 because, then, , and it is similar to the curve in the second case if ab < 0, because, then, ; in all the cases where a and b are not zero, the spiral therefore has a symmetry axis, like the Archimedean spiral; but the two infinite branches have asymptotes: the logarithmic spirals.

After this website, there are also cases where the ring oscillate on the rod ; the curve is so an epitrochoid.

These spirals are inverses of Poinsot spirals.
See also the spiral of the hyperbolic tangent.

© Robert FERRÉOL  2022