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In an Atwood machine the weight difference drives the system while the total mass resists it, so a = (m1 - m2)g/(m1 + m2). The rope tension sits between the two weights at 2.m1.m2.g/(m1+m2). With 8 kg against 5 kg the acceleration is 2.26 m/s2 and the tension 60.3 N - more than the light weight, less than the heavy one.
Acceleration
a = (m1 - m2) g / (m1 + m2)
Tension
T = 2 m1 m2 g / (m1 + m2)
Because an ideal rope has no mass and the pulley no friction. A real heavy rope or a stiff bearing breaks that assumption.
The acceleration is zero and the tension equals either weight - the system will stay wherever you leave it.