How do I calculate this about the z-axis, if the cuboid length is b in the y-direction, a in the x-direction and c in the z -direction?(adsbygoogle = window.adsbygoogle || []).push({});

In my notes, I have I = ∫ r^2 dm = ∫ (x^2 + y^2) dm

dm = ρdV = ρdxdydz

This is what I did:

I = ∫∫∫ (x^2 + y^2)ρ dxdydz

I = ρ∫dz∫dy∫dx (x^2 + y^2)

I = ρ∫∫dy [(1/3)x^3 + xy^2] {0->a}

I = ρ∫∫[(1/3)a^3 + ay^2] dy

I = ρ∫dz [(1/3)ya^3 + (1/3)ay^3] {0->b}

I = ρ∫dz [(1/3)ba^3 + (1/3)ab^3]

I = ρ[(1/3)zba^3 + (1/3)zab^3] {0->c}

I = ρ[(1/3)cba^3 + (1/3)cab^3]

I = (1/3)ρabc(a^2 + b^2)

and ρabc = ρV = M, so I = (1/3)M(a^2 + b^2)

However, the answer has (1/12) instead of (1/3). Where does the other 1/4 come from??

Thanks.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Moment of inertia of a cuboid

**Physics Forums | Science Articles, Homework Help, Discussion**