Question

http://www.feynmanlectures.info/exercises/atwood.html

Why can't i substitute the masses by an unique mass m, and just make F=mg? This way, if i apply Torricelli's equation, the result is g=v²/(2h)

Answer 1

The net force is F = m g

It produces work = change in kinetic energy of

m g h = (1/2) (2M+m) v^2

solve this for g and get the right answer.

Answer 2

Note that all three masses accelerate and then contribute to the kinetic energy