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The block labeled "a" has two radial forces on it. One, called $F_\textrm{ta}$, is directed toward the center, and provides the centripetal force needed for the mass to travel in a circle, but it is somewhat reduced by the other radial force, called $F_\textrm{tb}$, which is directed away from the center. The force toward the center is positive, whereas since the force $F_\textrm{tb}$ is directed away from the center, it subtracts from the centripetal force. The net force toward the center (and by net I mean after adding and subtracting all the radial forces) is always $\dfrac{mv^2}{r}$, and the subscript "a" is on each factor since the mass, velocity, and radius are for mass "a" in this case.