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19.4: Variations

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    In Sections 19.1,2,3, we imagined that the cycloid was generated by a circle that was rolling counterclockwise along the line \(y = 2a\). We can also imagine variations such as the circle rolling clockwise along \(y = 0\), or we can start with P at the top of the circle rather than at the bottom. I summarise in this section four variations. The distinction between \( \psi \) and \( \theta \) is as follows. The angle that the tangent to the cycloid makes with the positively-directed \(x\)-axis is \( \psi \); that is to say, \( dx/dy = tan \psi \).The circle rolls through an angle \(2 \theta \). There is a simple relation between \( \psi \) and \( \theta \), which is different for each case.

    In each figure, \(x\) and \(y\) are plotted in units of \(a\). The vertical height between vertices and cusps is \(2a\), the horizontal distance between a cusp and the next vertex is \( \pi a\), and the arc length between a cusp and the next vertex is \(4a\).

    I. Circle rolls counterclockwise along \(y = 2a\). P starts at the bottom. The cusps are up. A vertex is at the origin.

    \[\begin{align*} x &= a(2 \theta + \sin 2 \theta ) \\[4pt] y &= 2 a \sin^2 \theta \label{19.4.2}\tag{19.4.2} \\[4pt] s &= 4a \sin \theta \label{19.4.3}\tag{19.4.3} \\[4pt] ^2 &= 8 ay \label{19.4.4}\tag{19.4.4} \\[4pt] \psi &= \theta . \label{19.4.5}\tag{19.4.5} \end{align*} \]

    alt

    II. Circle rolls clockwise along \(y = 0\). P starts at the bottom. The cusps are down. A cusp is at the origin.

    \[ x = a(2\theta - sin 2\theta ) \label{19.4.6}\tag{19.4.6} \]

    \[ y = 2a sin^2 \theta \label{19.4.7}\tag{19.4.7} \]

    \[ s = 4a(1-cos \theta) \label{19.4.8}\tag{19.4.8} \]

    \[ s^2 = 8a(y-s) \label{19.4.9}\tag{19.4.9} \]

    \[ \psi = 90 \circ - \theta. \label{19.4.10}\tag{19.4.10} \]

    alt

    III. Circle rolls clockwise along \(y = 0\). P starts at the top. The cusps are down. A vertex is at \(x = 0\).

    \[ x = a( 2 \theta + \sin 2 \theta \label{19.4.11}\tag{19.4.11} \]

    \[ y = 2 a \cos^2 \theta \label{19.4.12}\tag{19.4.12} \]

    \[ s = 4 a \sin \theta \label{19.4.13}\tag{19.4.13} \]

    \[ s^2 = 8a(2a-y) \label{19.4.14}\tag{19.4.14} \]

    \[ \psi = 180 \circ - \theta . \label{19.4.15}\tag{19.4.15} \]

    alt

    IV. Circle rolls counterclockwise along \(y = 2a\). P starts at the top. The cusps are up. A cusp is at \(x = 0\).

    \[ x = a (2 \theta - \sin 2 \theta) \label{19.4.16}\tag{19.4.16} \]

    \[ y = 2a \cos^2 \theta \label{19.4.17}\tag{19.4.17} \]

    \[ s = 4a (1-\cos \theta) \label{19.4.18}\tag{19.4.18} \]

    \[ s^2 - 8as + 8a(2a - y) = 0 \label{19.4.19}\tag{19.4.19} \]

    \[ \psi = 90 \circ + \theta \label{19.4.20}\tag{19.4.20} \]

    alt


    This page titled 19.4: Variations is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jeremy Tatum via source content that was edited to the style and standards of the LibreTexts platform.