Dimensions of \(r=[L]\)
Dimensions of \(v=\left[L T^{-1}\right]\)
Dimensions of \(l=[L]\)
\(\eta=\frac{P\left(r^{2}-x^{2}\right)}{4 v l}\)
\(=\frac{\left.\left[M L^{-1} T^{-2}\right] L^{2}\right]}{\left[L T^{-1}\right][L]}\)
\(=\left[M L^{-1} T^{-1}\right]\)