Find Angular Acceleration: Torque & Rotational I

[wushumaster]

Homework Statement

The figure shows a view from above of two objects attached to the end of a rigid massless rod at rest on a frictionless table. Imagine a vertical rod, on the top end is a mass of m, an the bottom is a mass of 2m. The length of the rod is L and L/4 down from m (at top), a horizontal and rightward force is applied. When this F is applied, the resulting rotational acceleration of the rod about its center of mass is kF/(mL).

Homework Equations

What is K?

The Attempt at a Solution

So I found the center of mass as 2/3 L below mass m. but I don't understand how we can get torque (like I know its r cross F), but.. We can use torque=I times angular acceleration. But i got I wrong. HELP! please!

 

[wushumaster]

oh! is it just because torque equals r cross f and that means it equals 2/3 L- 1/4 L?