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# Constant-speed ramps for a central force field

• * Corresponding author
Rafael López has partially supported by the grant no. MTM2017-89677-P, MINECO/AEI/FEDER, UE
• We investigate the problem of determining the planar curves that describe ramps where a particle of mass $m$ moves with constant-speed when is subject to the action of the friction force and a force whose magnitude $F(r)$ depends only on the distance $r$ from the origin. In this paper we describe all the constant-speed ramps for the case $F(r) = -m/r$. We show the circles and the logarithmic spirals play an important role. Not only they are solutions but every other solution approaches either a circle or a logarithmic spiral.

Mathematics Subject Classification: 70E18, 53A17.

 Citation: • • Figure 1.  A mass $M$ sliding along $\alpha$ under the effect of a central force ${\mathbf F}({\mathbf r})$ and the friction force

Figure 2.  Case $v = 1$. The phase portrait of the system (17), with $\mu = 0.5$. The origin $(0,0)$ is the only equilibrium point and it is a stable focus

Figure 3.  Case $v = 1$. The purple part of the logarithmic spiral (left) is the TreadmillSled of the non-circular ramp (right)

Figure 4.  The phase portrait of the system (21). Left: $v = 2$ and $\mu = 0.1$. Right: $v = 0.5$ and $\mu = 0.3$

Figure 5.  TreadmillSleds that are half-lines. Left: $v = 2$, $\mu = 0.1$. Right: $v = 0.5$, $\mu = 0.3$

Figure 6.  Constant-speed ramps whose TreadmillSleds are half-lines of Figure 5, left. Here $v = 2$, $\mu = 0.1$. Left: parametrization (23) for $\mathbf{a}$. Right: parametrization (23) for $-\mathbf{a}$

Figure 7.  Constant-speed ramps $\alpha_{ls}$ whose TreadmillSleds $\gamma_{ls}$ are the half-lines of Figure 5, right. Here $v = 0.5$ and $\mu = 0.3$. Left: parametrization (23) for $\mathbf{a}$. Right: parametrization (23) for $-\mathbf{a}$

Figure 8.  Constant-speed ramps whose TreadmillSleds are not half-lines. Here $v = 2$ and $\mu = 0.1$. Left: the TreadmillSled which is asymptotic to the line of vector $\mathbf{a}$. Right: the constant-speed ramp

Figure 9.  Constant-speed ramps whose TreadmillSleds are not half-lines. Here $v = 0.8$ and $\mu = 0.3$. Left: the TreadmillSled. Right: the constant-speed ramp

Figure 10.  Constant-speed ramps that are not spirals. Left: $v = 2$ and $\mu = 0.1$. Right: $v = 0.8$ and $\mu = 0.3$

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