Spirals - Archimedean, logarithmic, hyperbolic and clothoid

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Find the derivative yx′y'_{x} of the following implicitly defined function:arctg⁡yx=ln⁡x2+y2\arctg \dfrac {y}{x}=\ln \sqrt{x^{2}+y^{2}} (logarithmic spiral).

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Find the radius of curvature of the curve given by the following polar equation: the Archimedean spiral r=aφr=a\varphi.

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Find the radius of curvature of the curve given by the following polar equation: the logarithmic spiral r=aemφr=ae^{m\varphi }.

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Find the angle between the tangent and the polar radius of the point of tangency for the following curves:

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(1)

the spiral of Archimedes r=aφr = a \varphi;

(2)

the hyperbolic spiral r=a/φr = a / \varphi;

(3)

the logarithmic spiral r=aebφr = a e^{b \varphi };

(4)

the cardioid r=a(cos⁡φ+1)r = a(\cos \varphi +1);

(5)

the arc of the lemniscate of Bernoulli r2=a2cos⁡2φ,0⩽φ<π/4r^{2} = a^{2} \cos 2 \varphi , 0 \leqslant \varphi < \pi / 4.

Find the condition on the parameter aa of the logarithmic spiral ρ=caφ\rho = c a^{\varphi } under which its evolute coincides with the spiral itself.

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Prove that for points of the Archimedean spiral ρ=aφ\rho = a \varphi, as φ→+∞\varphi \rightarrow +\infty, the difference between the length of the radius vector and the radius of curvature tends to zero.

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Prove that under the conditions of Problem 94 the center of curvature moves along a curve tending to coincide with the circle ρ=1\rho = 1.

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Prove that the centers of curvature at points of the Archimedean spiral ρ=aφ\rho = a \varphi lying on one ray are located on an ellipse whose semi-axes do not depend on the choice of ray.

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Plot the graph of the function in polar coordinates:

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(1)

r=1/φr = 1 / \varphi (hyperbolic spiral);

(2)

r=eφr = e^{\varphi } (logarithmic spiral);

(3)

r=8sin⁡(φ−π/3)r = 8 \sin (\varphi -\pi / 3);

(4)

r=1/(1−sin⁡φ)r = 1 /(1-\sin \varphi ).

Find the area of the sector φ1⩽φ⩽φ2,φ2−φ1⩽2π\varphi_{1} \leqslant \varphi \leqslant \varphi_{2}, \varphi_{2}-\varphi_{1} \leqslant 2 \pi, bounded by the curve:

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(1)

r=aφ/(2π)r = a \varphi /(2 \pi ) (Archimedean spiral);

(2)

rφ=ar \varphi = a (hyperbolic spiral);

(3)

r=Rekφ,k>0r = R e^{k \varphi }, \quad k > 0 (logarithmic spiral).

Find the area of the figure bounded by the nn-th turn of the Archimedean spiral

r=aφ/(2π),2π(n−1)⩽φ⩽2πn,n∈N r = a \varphi /(2 \pi ), \quad 2 \pi (n-1) \leqslant \varphi \leqslant 2 \pi n, \quad n \in \mathbb {N}

and a segment of the polar axis.

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Find the area of the figure bounded by a segment of the polar axis and two turns of the spiral corresponding to the values of the polar angle φ∈[2πn;2π(n+1)]\varphi \in [2 \pi n ; 2 \pi (n+1)] and φ∈[2π(n+1);2π(n+2)]\varphi \in [2 \pi (n+1) ; 2 \pi (n+2)]:

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(1)

r=a2πφr = \frac{a}{2 \pi } \varphi

(2)

rφ=ar \varphi = a;

(3)

r=Rekφ,k>0r = R e^{k \varphi }, \quad k > 0.

Find the length of the curve:

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(1)

x=∫0tcos⁡φ2dφ,y=∫0tsin⁡φ2dφ,0⩽t⩽t0x = \int_{0}^{t} \cos \varphi^{2} d \varphi , \quad y = \int_{0}^{t} \sin \varphi^{2} d \varphi , \quad 0 \leqslant t \leqslant t_{0} (clothoid);

(2)

x=∫1tsin⁡φφdφ,y=∫1tcos⁡φφdφ,1⩽t⩽t0x = \int_{1}^{t} \frac{\sin \varphi }{\varphi } d \varphi , \quad y = \int_{1}^{t} \frac{\cos \varphi }{\varphi } d \varphi , \quad 1 \leqslant t \leqslant t_{0}.

Find the length of the arc of the curve:

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(1)

r=asin⁡φr = a \sin \varphi

(2)

r=aekφ,φ1⩽φ⩽φ2r = a e^{k \varphi }, \quad \varphi_{1} \leqslant \varphi \leqslant \varphi_{2} (logarithmic spiral);

(3)

r=a(1−cos⁡φ)r = a(1-\cos \varphi ) (cardioid);

(4)

r=2(1+cos⁡φ),r⩽1r = 2(1+\cos \varphi ), \quad r \leqslant 1;

(5)

r=a(1−sin⁡φ),−π/2⩽φ⩽−π/6r = a(1-\sin \varphi ), \quad -\pi / 2 \leqslant \varphi \leqslant -\pi / 6;

(6)

r=ath⁡(φ/2),0⩽φ⩽φ0r = a \operatorname {th}(\varphi / 2), \quad 0 \leqslant \varphi \leqslant \varphi_{0}.

Prove that the lengths of the arcs of successive turns of the logarithmic spiral

r=aekφ,2πn⩽φ⩽2π(n+1) r = a e^{k \varphi }, \quad 2 \pi n \leqslant \varphi \leqslant 2 \pi (n+1)

form a geometric progression, and find its common ratio.

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Let s(α)s(\alpha ) be the length of the arc of the logarithmic spiral

r=aekφ,k>0,α⩽φ⩽0. r = a e^{k \varphi }, \quad k > 0, \quad \alpha \leqslant \varphi \leqslant 0.

Find lim⁡α→−∞s(α)\lim_{\alpha \rightarrow -\infty } s(\alpha ).

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Find the length of the arc of the curve:

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(1)

r=aφ,0⩽φ⩽φ0r = a \varphi , \quad 0 \leqslant \varphi \leqslant \varphi_{0}

(2)

r=aφ2,0⩽φ⩽4r = a \varphi^{2}, \quad 0 \leqslant \varphi \leqslant 4;

(3)

r=aφ4,0⩽φ⩽3r = a \varphi^{4}, \quad 0 \leqslant \varphi \leqslant 3;

(4)

r=aφ3,0⩽φ⩽4r = a \varphi^{3}, \quad 0 \leqslant \varphi \leqslant 4.

(5)

(Archimedean spiral.) A ray rotates about a point OO with constant angular velocity ω\omega. A point MM moves along it with constant speed vv. Set up the equation, in polar coordinates, of the curve traced by the point MM, if at the initial moment of motion the ray coincides with the positive direction of the abscissa axis, and the point MM with the origin OO. Sketch the resulting curve.

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Prove that in the preceding problem the Archimedean spiral is tangent to the rotating ray at the initial moment of time.

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Prove that the Archimedean spiral is not an algebraic curve.

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References

[Dem97] Demidovich B.P. Сборник задач и упражнений по математическому анализу. 13-е изд., Издательство Московского университета / Издательство "ЧеРо", 1997.
[ККЧ+03a] Кудрявцев Л.Д., Кутасов А.Д., Чехлов В.И., Шабунин М.И. Предел. Непрерывность. Дифференцируемость. 2-е изд., ФИЗМАТЛИТ, 2003.
[ККЧ+03b] Кудрявцев Л.Д., Кутасов А.Д., Чехлов В.И., Шабунин М.И. Интегралы. Ряды. 2-е изд., ФИЗМАТЛИТ, 2003.
[Сми16] Смирнов Ю.М. Сборник задач по аналитической геометрии и линейной алгебре. МЦНМО, 2016.