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I am trying to do a problem from Arnold; Mathematical methods of Classical mechanics. But I didn't get the desired result mentioned by the author.

Let $E_0$ be the value of the potential function at a minimum point $\xi$. Find the period $T_0 = \lim_{E\to E_0} T(E)$ of small oscillations in a neighbourhood of the point $\xi$.

Answer: $\frac{2\pi}{\sqrt{U''(\xi)}}$, Where $U(\xi)$ is the potential energy.

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    $\begingroup$ Hint: The period is just $P=2 \pi/ \omega$, where $ \omega$ is the pulsation. For a potential of the form $U=U'' x^2/2$... (the mass is 1). $\endgroup$
    – Quillo
    Jun 14, 2022 at 6:36

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It's an important result with a large scope.

Let $U$ be the potential with a minimum for $x=\xi$. Taylor expansion near $\xi$ is:

$$U(x)\simeq U(\xi)+(x-\xi)U'(\xi)+\frac{1}{2}(x-\xi)^2U''(\xi)$$

Since $\xi$ is the location of a minimum, you have both:

$$U'(\xi)=0\\ U''(\xi)>0$$

Conservation of mechanical energy yields, with $k=U''(\xi)$:

$$\frac{d}{dt}\left(\frac{1}{2}\,m\dot{x}^2+U(x)\right)=0 \quad\Rightarrow\quad m\ddot{x}+k(x-\xi)=0 $$

You can read directly in this harmonic oscillator equation the angular frequency:

$$\omega^2=\frac{k}{m}$$

and the associated period:

$$T=\frac{2\pi}{\omega}=2\pi\sqrt{\frac{m}{U''(\xi)}}$$

$m$ is missing in the result you mention. Perhaps $U$ is redefined to contain $m$, or $U$ is a gravitational potential instead of a potential energy. But you get the idea.

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  • $\begingroup$ Thanks a lot, thanks for the simple explanation @Miyase $\endgroup$ Jun 14, 2022 at 8:13

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