Clarity on Intuition on simple mass pulley problem

Question

I was solving some simple pulley problems and needed some clarity on the intuition i had to improve my speed in competitive exams.
Solving through newton's equations i do get the correct answer that is 4.8 kgs for the mass M.
I had initially thought that if the Mass M were to be 5kgs then the net external unbalanced force due to gravity on both the sides would be 5kgs thus making the system to be in rest.
Solving through equations, the answer 4.8 looks fully correct. But i dont know why my intuition doesnt work on this particular question when i look at the system from the whole. Tension being an internal force should cancel out so doesnt that just leave gravity. Does it have something to do with pseudoforce.

Farcher · Answer

What you have to appreciate is that whole system of pulleys and masses cannot be in static equilibrium as the $3,rm kg$ mass must accelerate downwards whilst the $2,rm kg$ must accelerate upwards.
That being the case the tension in the string around the lower pulley must be smaller than $3g ,rm N$ so that the $3,rm kg$ mass accelerates downwards and larger than $2g ,rm N$ so that the $2,rm kg$ mass accelerates upwards.
This means that the tension in the string around the top pulley must be less than $5g,rm N$ and so mass $M$ must be less that $5,rm kg$ so that that section of the system is in static equilibrium.

Clarity on Intuition on simple mass pulley problem

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