The angular momentum of the particle is defined as the moment of momentum about the axis of rotation.

∴ Angular momentum = linear momentum x perpendicular distance from the axis.

If ω is the angular velocity of the particle, then the linear velocity v = r ω.

∴ Angular momentum of each particle = (mv)r = (mrω) r = mr^{2}ω.

∴ Total angular momnetum of the body is L = (Σ mr^{2}ω) = (Σ mr^{2})

ω = lω where l = Σ mr^{2}.