# Finding the moment of inertia of a compound object

1. Nov 12, 2009

### dentulousfing

1. The problem statement, all variables and given/known data
A uniform bas has two small balls glued to its end, the bar is 2.00 m long, and it has a mass 4.00 Kg, while the balls, have mass 0.500Kg and can be treated as point masses. Find the moment of inertia of this combination about each of the following axes. a) an axis perpendicular to the bar through its center.

2. Relevant equations
I=1/3ML^2
I=$$\Sigma$$mr^2

3. The attempt at a solution
I=1/3ML^2
I=1/3(4.00Kg)(2.00m)^2
I=5.33 Kgm^2

I=$$\Sigma$$mr^2
I=(0.500Kg)(2.00m)^2(2)
I=1.0Kgm^2

I tot =6.33 Kgm^2
I am supposed to add both results?, how is the result of hit sexercise 2.33 Kgm^2?
thanks.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 12, 2009

### rl.bhat

I of bar is 1/12*M*L^2.

3. Nov 12, 2009

### dentulousfing

I believe that's the equation when the axis of rotation goes through one of the bar's end. when the axis crosses the center of the bar I=1/3ML^2 according to my text book.

4. Nov 12, 2009

### dentulousfing

you were right, I am so sorry. thanks. I finally got it.!!