Trochoid

File:TrohoidH1,25.gif

File:TrohoidH0,8.gif

As a circle of radius {{mvar|a}} rolls without slipping along a line {{mvar|L}}, the center {{mvar|C}} moves parallel to {{mvar|L}}, and every other point {{mvar|P}} in the rotating plane rigidly attached to the circle traces the curve called the trochoid. Let {{math|1={{overline|CP}} = b}}. Parametric equations of the trochoid for which {{mvar|L}} is the {{mvar|x}}-axis are

:$\backslash begin\{align\}$

& x = a\theta - b \sin \theta \\

& y = a - b \cos \theta

\end{align}

where {{mvar|θ}} is the variable angle through which the circle rolls.

If {{mvar|P}} lies inside the circle ({{math|b < a}}), on its circumference ({{math|1=b = a}}), or outside ({{math|b > a}}), the trochoid is described as being curtate ("contracted"), common, or prolate ("extended"), respectively.{{Cite web|title = Trochoid|website = Xah Math|url = http://www.xahlee.org/SpecialPlaneCurves_dir/Trochoid_dir/trochoid.html|accessdate = October 4, 2014}} A curtate trochoid is traced by a pedal (relative to the ground) when a normally geared bicycle is pedaled along a straight line.{{cite AV media| url-status = live| archive-url = https://ghostarchive.org/varchive/youtube/20211211/aJhiY70KY5o| archive-date = 2021-12-11| url = https://www.youtube.com/watch?v=aJhiY70KY5o| title = The Bicycle Pulling Puzzle | website=YouTube}}{{cbignore}} A prolate trochoid is traced by the tip of a paddle (relative to the water's surface) when a boat is driven with constant velocity by paddle wheels; this curve contains loops. A common trochoid, also called a cycloid, has cusps at the points where {{mvar|P}} touches the line {{mvar|L}}.

A more general approach would define a trochoid as the locus of a point $(x,y)$ orbiting at a constant rate around an axis located at $(x\text{'},y\text{'})$,

:$x=x\text{'}+r\_1\backslash cos(\backslash omega\_1\; t+\backslash phi\_1),\backslash \; y=y\text{'}+r\_1\backslash sin(\backslash omega\_1\; t+\backslash phi\_1),\backslash \; r\_1>0,$

which axis is being translated in the x-y-plane at a constant rate in either a straight line,

:$\backslash begin\{array\}\{lcl\}$

x'=x_0+v_{2x} t,\ y'=y_0+v_{2y} t\\

\therefore x = x_0+r_1\cos(\omega_1 t+\phi_1)+v_{2x} t,\ y = y_0+r_1 \sin(\omega_1 t+\phi_1)+v_{2y} t,\\

\end{array}

or a circular path (another orbit) around $(x\_0,y\_0)$ (the hypotrochoid/epitrochoid case),

:$\backslash begin\{array\}\{lcl\}$

x' = x_0+r_2\cos(\omega_2 t+\phi_2),\ y' = y_0+r_2\sin(\omega_2 t+\phi_2),\ r_2\ge 0\\

\therefore x = x_0+r_1\cos(\omega_1 t+\phi_1)+r_2\cos(\omega_2 t+\phi_2),\ y = y_0+r_1 \sin(\omega_1 t+\phi_1)+r_2\sin(\omega_2 t+\phi_2),\\

\end{array}

The ratio of the rates of motion and whether the moving axis translates in a straight or circular path determines the shape of the trochoid. In the case of a straight path, one full rotation coincides with one period of a periodic (repeating) locus. In the case of a circular path for the moving axis, the locus is periodic only if the ratio of these angular motions, $\backslash omega\_1/\backslash omega\_2$, is a rational number, say $p/q$, where $p$ & $q$ are coprime, in which case, one period consists of $p$ orbits around the moving axis and $q$ orbits of the moving axis around the point $(x\_0,y\_0)$. The special cases of the epicycloid and hypocycloid, generated by tracing the locus of a point on the perimeter of a circle of radius $r\_1$ while it is rolled on the perimeter of a stationary circle of radius $R$, have the following properties:

:$\backslash begin\{array\}\{lcl\}$

\text{epicycloid: }&\omega_1/\omega_2&=p/q=r_2/r_1=R/r_1+1,\ |p-q| \text{ cusps}\\

\text{hypocycloid: }&\omega_1/\omega_2&=p/q=-r_2/r_1=-(R/r_1-1),\ |p-q|=|p|+|q| \text{ cusps}

\end{array}

where $r\_2$ is the radius of the orbit of the moving axis. The number of cusps given above also hold true for any epitrochoid and hypotrochoid, with "cusps" replaced by either "radial maxima" or "radial minima".

==See also==

• Aristotle's wheel paradox
• Brachistochrone
• Cyclogon
• Cycloid
• Epitrochoid
• Hypotrochoid
• List of periodic functions
• Roulette (curve)
• Spirograph
• Trochoidal wave

{{reflist}}

• [http://jsxgraph.uni-bayreuth.de/wiki/index.php/Trochoid Online experiments with the Trochoid using JSXGraph]

Category:Roulettes (curve)