Circular Motion question, why does Fnet = 0 (so that T=W)?

[sodnaz]

Homework Statement

A 0.50kg puck rests on a level air table and is connected by a light thread passing through a hole in the table to support a hanging mass of 3.0 kg. A stable orbit is achieved when the puck is sent into a circular path of radius 0.15 metres around the hole.
(A) neglecting friction at the edge of the hole, calculate the period of revolution of the puck in its orbit.

Homework Equations

Centripetal force = Tension - Weight
V=2πr/T

The Attempt at a Solution

I know that at that position, F = T-W

I saw this solution online:

However, I don't get why Fnet (centripetal force) = 0 in this instance? If the puck is in it's orbit, how can the centripetal force be 0? Can you explain that please? I just don't understand why the centripetal force is 0?

Thanks

 

[Chestermiller]

Have you drawn a free body diagram on the hanging 3 kg mass? If so, what is the force balance equation on this mass? What is the tension in the thread?

Have you drawn a free body diagram on the on the 0.5 kg mass? If so, what is the force balance equation on this mass? What is the radial acceleration of this mass?