(U = mgy) with (y = -L\cos\theta) gives (U = -mgL\cos\theta).

Simple pendulum equation: ( \ddot\theta + (g/l)\sin\theta = 0 ).

A massless, frictionless pulley with two masses (m_1) and (m_2) connected by a massless string of fixed length. Let (x) be the height of (m_1) below the pulley axle.

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