Question #b0e5d

1 Answer
Nov 13, 2017

It is so to keep the problem simple. Total mechanical energy in that case is only distributed between the two hanging masses. If pulley has a non-negligible mass its energy must also be included in predicting the dynamics.


Atwood machine problem is very simple if you choose a pulley of negligible mass. If the pulley mass is not negligible then you must include the rotational energy of the pulley to be able to apply the mechanical energy conservation condition.

The problem is solvable but this makes it a complicated problem. It can no longer be used to demonstrate the power of mechanical energy conservation condition to a juvenile physics student. After a student advances, may be they can relax this condition of negligible pulley mass and repeat the exercise of predicting its dynamics.