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Consider a particle P moving in a circle of radius r as shown in the figure.

Premise 1: Position of the particle can be described by the angle $\theta$.
enter image description here

Premise 2: Particle reaches the position with an angle $\theta$ if it has covered the angle lesser than $\theta$.

Premise 3: Particle can never reach non-terminating decimal form radian value.
For example: 0.33333....rad; The particle can never reach this radian value because, to reach this value, first particle needs to cover 0.33 rad, then 0.333 rad, then 0.33333, and so on. Particle needs to go on and on, and the sequence of 0.333.. never ends.

Premise 4: As the particle can never reach non-terminating decimal form radian value, it can't even reach neighbor terminating decimal form radian value. For example 0.4 (considering 0.333.. of Premise 3) can never be reached as 0.333... can never be reached.

The argument, as evident, is in the zeno version. But, I am not able to come out of this cage. What is going wrong in this argument? Can the particle never reach non-terminating decimal form value? I think this problem can be generalized to all the physical quantities.

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  • $\begingroup$ Your question doesn't make any sense to me: The point of a circle is that every point is the same (heh... point), so what is stopping us from setting up a radial coordinate $\theta$ such that the position of the particle at $t=0$ is given by $\theta=\pi$? This shows that your entire argument cannot be any good, because a similarly "tempting" fallacy cannot be made in this case. $\endgroup$
    – Danu
    Commented Apr 3, 2015 at 9:23
  • $\begingroup$ @Danu: The point of a circle is that every point is the same (heh... point), so what is stopping us from setting up a radial coordinate θ such that the position of the particle at t=0 is given by θ=π?- Sorry, I didn't get your point (essential meaning). $\endgroup$
    – Sensebe
    Commented Apr 3, 2015 at 9:36
  • $\begingroup$ What's the matter in summing up an infinite amount of (small) steps such that you obtain a finite amount in the end? This is the basis of measure theory/integration if you want, or of the fact that infinite series may converge. The solution of zeno paradoxes is always, roughly speaking, the same: you can do an infinite number of operations in a finite time, provided each operation takes a smaller and smaller amount of time to be made. $\endgroup$
    – yuggib
    Commented Apr 3, 2015 at 10:04
  • $\begingroup$ What yuggib said is the real (commonly accepted) solution to the paradox. I was just pointing out that there is no real meaning to the statement that a particle has a position with irrational value of the coordinate, since you can choose the coordinate to pretty much say anything (for instance, say that the particle starts at $\pi$). This shows that one cannot make any physical claims based on the value that your coordinate reads off. $\endgroup$
    – Danu
    Commented Apr 3, 2015 at 10:25

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