Find Net Torque: Solve Homework Problem w/ F1, F2, F3, F4, r, R

[lemonpie]

Homework Statement

A cylinder having a mass of 2.0kg can rotate about its central axis through point O. Forces are applied as show: F1 = 6.0N, F2 = 4.0N, F3 = 2.0 N, and F4 = 5.0N. Also, r = 5.0cm and R = 12 cm. Find the (a) magnitude and (b) direction of the angular acceleration of the cylinder.
http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c10/fig10_42.gif

Homework Equations

The Attempt at a Solution

as of yet, I'm not looking for the solution to the entire problem, because I'm stuck on the first step of finding the net torque. this is what i have:

net torque = F1R - F2R - F3r + F4R = 6*0.12 - 4*0.12 - 2*0.05 + 0 = 0.14 N m

the book's solution for the net torque is 71 N m. i also got a similar problem wrong for presumably the same reason. the units are Newtons and meters, and I'm sure my calculator works fine. what is going on here?

 

[rl.bhat]

torque = I*alpha.
Find the moment of inertia of the cylinder and then find angular acceleration.

 

[lemonpie]

thanks... but can you please tell me why what i was doing was wrong?

 

[Dweirdo]

F4 is considered as torque with respect to the center?
oh just saw that You wrote 0, my bad.
Whati s the answer in the book for the angular acc?

 

[lemonpie]

Homework Statement

net torque = F1R - F2R - F3r + F4R = 6*0.12 - 4*0.12 - 2*0.05 + 0 = 0.14 N m

i'd like to go back to this step, because this is what i found and this is what was written in the student solutions manual, EXCEPT the answer turned out to be 71 N m instead of 0.14 N m and i can't see how that could possibly be the case. I'm also getting other problems wrong at this step, and i can't move forward until i see what I'm doing wrong here, so if anyone could explain to me this particular step, instead of how to solve the entire thing, that would be a really big help to me. thanks!