This question comes from the following website: https://i-want-to-study-engineering.org/q/pulley_dynamics_1/
A system consists of three light pulleys and light inextensible strings as shown in the figure. If the upward accelerations of A and D are 4m/s^2 and −4m/s^2 respectively and B is not accelerating relative to the global coordinate system, calculate the upward acceleration of C.
The video explanation for the solution starts off by stating:
The left hand side of this system has two constraints. Mass A has an acceleration of 4m/s^2 and mass B has zero acceleration. Looking at Point P, these constraints mean that for every 2 units that mass A is elevated by, the point P increases in height by 1 unit. Hence the absolute acceleration of the connecting string is 2m/s^2
I am afraid that I do not understand how the acceleration of masses A and B being 4 and 0 respectively implies that their combined absolute acceleration is 2.
Is the absolute acceleration of A and B not already 4 and 0? I am bamboozled.