# Moment of Inertia, working backwards

1. Oct 21, 2014

### Niall

I've to work out b & d in this equasion.

I=bd^3/12

I know that the ratio between b & d in this case is 1.166 and im given the value of I.

It is an engineering problem as the software will only allow the use of square/rectangular or round section but we are using H section and we are allowed to substitue a rectangle of the same h/w ratio and moment of inertia.

This kind of thing was never what I was much good at although I believe it may be possible to work out but I dont know how.
Thanks

2. Oct 21, 2014

### Staff: Mentor

Do you mean $\frac{b}{d} = 1.166$? Then $b = 1.166d$, put that into your equation for the moment of inertia, and solve for $d$.

3. Oct 21, 2014

### Niall

Yes , I managed to figure it out a few min ago.

Ixx = (D (1.116) * D ^ 3) / 12

D^4 = I * 12 * 1.116

D = 4√ I * 12 * 1.116

4. Oct 21, 2014

### HallsofIvy

In your original post you used "d". In your response you used "D". It is a bad idea to switch symbols like that.