Moment of Inertia of 4 Balls on Square Side

[erisedk]

Homework Statement

There are four solid balls with their centres at the four corners of a square of side a. The mass of each sphere is m and radius is r. Find the moment of inertia of the system about one of the sides of the square.

Homework Equations

The Attempt at a Solution

I= 2 (two balls) × [ 2/5 mr^2 (Icm) + ma^2 ] (just used parallel axis theorem)

However, in the answer,
I = 2 ( 2/5 mr^2 ) + 2×[ 2/5 mr^2 + ma^2 ]

Why have they added 2 ( 2/5 mr^2 ) ??

 

[BvU]

Using the parallel axis theorem is good. I don't see it under 2. What does it look like and for what balls do you apply it ?

 

[haruspex]

There are four solid balls

Why have they added 2 ( 2/5 mr^2 ) ??

Consider the above two lines.

 

[erisedk]

Ok, right!
I got it, thanks. I didn't consider moi of the two balls along the axis.