Atwoods Machine Problems: describes a method for solving atwood machine problems when the pulley's mass is negligible.
This video assumes the mass is negligible (i.e., the rotational inertia of the pulley is near zero). It can also represent a problem with a massive pulley that offers no friction between itself and the string so that the pulley does not turn. For a video that gives the details of what to do when the pulley is massive see    • Rotational Dynamics (part III)   .
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