# Moment of Inertia of L-shaped bar

1. Dec 1, 2007

### roldy

1. The problem statement, all variables and given/known data
Find the mass moment of inertia of a L-shaped bar that has legs of 1.2 m long and total mass of bar is 20 kg.

2. Relevant equations
I=1/12*m*L^2
parallel axis theorem

3. The attempt at a solution
I've tried attempting this problem but I just can't figure out how to apply mass moments to the parallel axis theorem.

Any help on this problem would be greatly appreciated. I need to figure this mass moment of inertia for a rotational dynamics problem I'm working on.

2. Dec 1, 2007

### Staff: Mentor

Add the moment of inertia of each "leg" together to get the total. Where's the axis of rotation?

The formula you gave is for a thin rod rotating about its center. You can use the parallel axis theorem to find the moment of inertia of a thin rod about any parallel axis.

3. Dec 3, 2007

### roldy

still no luck

I do not know where the axis of rotation is. The problem states that a slender L-shaped bar is at rest in a vertical plane.

a bar rotating about it's end is 1/3mL^2

4. Dec 3, 2007

### Staff: Mentor

If you're not told the axis, how can you be expected to find the moment of inertia?

Please post the full problem exactly as given.

5. Dec 4, 2007

### roldy

I'll scan in the picture tomorrow along with the question