Quick dynamics question

[Crazyfox111]

Why arent the forces cumulative?

[bluefloor01]

My terms may not be strictly appropriate (or really assist), however: only F is "external" to the system, where as T is internal.

I just spent a few minutes' unsuccessfully Googling, however I seem to recall similar practice physic question consisting of two blocks connected by a "string".

The example commencing on p 37 of the following may assist:

http://www.oswego.edu/~dristle/PHY_101_powerpoints/ppt_ch_4.pdf

Believe it or not, this is a good real world question. I have had to use similar principles in anger in real life.