Obtain `tau=Ialpha` from angular momentum of rigid body.

Explanation

The total angular momentum of a motion of rigid body about a fixed axis
`vecL=vec(L_(z))+vec(L_(_|_))`
Differentiating w.r.t. time
`(dvecL)/(dt)=(dvec(L_(z)))/(dt)+(dvec(L_(_|_)))/(dt)`
but `(dvec(L_(z)))/(dt)=tauhatk and (dvec(L_(_|_)))/(dt)=0`
`therefore (dvec(L))/(dt)=tauhatk`
but `vec(L_(Z))=Iomegahatk`
`therefore (d(Iomega))/(dt)=tauhatk`
`I(domega)/(dt)=tau`
`therefore Ivecalpha=vectauimpliesvectau=Ivecalpha`
Scalar form `tau=Ialpha` is similar to `F=ma` in linear motion.