Tension constraint

I have a question about the pulley configuration in the following problem: Why can't we apply the tension constraint $$T_{1a}-T_{2a}-T_{1a}=0?$$ This would imply that $$T_{2a}=0$$. I know this is incorrect, and there are other simple ways to solve this question, but why can't we apply tension constraint? Please solve my doubt and make my concept clear.

• You need to state the basis of your proposed constraint. – Bob D Jul 24 at 15:19
• If the velocity vector is constant then the sum of products of all tensions in strings and accelerations of respective blocks connected to the strings is equal to 0. It is mathematically represented by : ∑T⋅a=0 – Physics freak Jul 24 at 15:25
• What do you mean by the velocity vector being constant? Clearly the blocks have to be accelerating. Also, why are you multiplying tension by acceleration? – Bob D Jul 24 at 15:34
• I am using the tension constraint. Constraint relation says that the sum of products of all tensions in strings and acceleration of respective blocks connected to the strings is equal to 0 – Physics freak Jul 24 at 15:39
• Sorry but it’s not making any sense to me that – Bob D Jul 24 at 15:46