In a double pulley system, if the mass of the object is 10 kg and the object is accelerating upwards at 2 m/s$^2$, what is the tension in the ropes?
I figured the weight of the box is 100N and the upward force is 20 N. Since these forces are in opposite direction I would think to subtract them and get 80N and divide it by the two ropes. 40N
However my textbook says $$2T =m(a+g)\\ 2T = 10kg (2m/s^2 + 10m/s^2)\\ 2T = 10kg \cdot 12m/s^2\\ 2T= 120N \\ T = 60N $$
Basically its saying that the 2 forces are added together. This does not make intuitive sense to me, even when looking at a diagram of a double pulley.
Can someone explain this to me.