1. The problem statement, all variables and given/known data Consider a situation I'm curious why is that the massless pulley's acceleration is just the avarage of acceleration of m1 and that of m2? ab=(a1+a2)/2 2. Relevant equations Forces acting on pulley are Tc-2Ta=0 (because pulley is massless, force can't give it acceleration) Therefore tensions are Tc=2Ta But, Ta-m1g=m1a1 and Ta-m2g=m2a2 3. The attempt at a solution Intuitively it makes sense.But I can't find the equation that describes this relationship.