How does tension affect the motion of a hanging block on a rotating disk?

[Brandan]

Homework Statement

This figure shows a uniform disk, with mass M = 2.5 kg
and radius R = 20 cm, mounted on a fixed horizontal axle.
A block with mass m = 1.2 kg hangs from a massless cord
that is wrapped around the rim of the disk.

Sample problem 10-8 (has picture diagram and detailed solution): http://astro1.panet.utoledo.edu/~mheben/PHYS_2130/Chapter11-1_mh.pdf

Homework Equations

T – mg = ma
torque = I α
-RT ? = (1/2)MR^2 α
a = a(subscript t) = α R

The Attempt at a Solution

I understand how to do the problem but what I don't understand is how they got -RT. Torque is FrSin(theta) and the sin of 90 degrees is 1 not -1 so I would really appreciate any insight on how they got -RT.

 

[lep11]

Well, it looks like positive direction is chosen to be up and vector T is pointing down so...

 

[Brandan]

Well, it looks like positive direction is chosen to be up and vector T is pointing down so...

Ok thanks