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Finding net torque

  • Thread starter lemonpie
  • Start date
  • #1
38
0

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?
 

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
5
torque = I*alpha.
Find the moment of inertia of the cylinder and then find angular acceleration.
 
  • #3
38
0
thanks... but can you please tell me why what i was doing was wrong?
 
  • #4
174
0
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?
 
  • #5
38
0

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!
 

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