I know what the parallel axis theorem is, but I'm a little confused about when to use it. I recently had a problem where a hoop was rolling down an incline where I used the parallel axis theorem to find the translational acceleration and got it correct. However, I had a problem about a spool being pulled by a string (think of a yo-yo being pulled on the ground), and when I set up the equations I got the wrong answer using the P.A.T. For instance, I had:(adsbygoogle = window.adsbygoogle || []).push({});

[tex]F-F_{S}=ma[/tex]

[tex]RF_{S}-rF=I_{CM}\alpha[/tex]

Why isn't the I_{CM}instead I_{P}? I have solved the problem already and know the answer, I just can't see why the parallel axis theorem is not used. LINK: http://show.imagehosting.us/show/971155/0/nouser_971/T0_-1_971155.jpeg

Thank you for your help.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Parallel Axis Theorem

**Physics Forums | Science Articles, Homework Help, Discussion**