 Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2017/paper-2/12/a/ii/solution

Height above the surface is

h = r - R

. Integrate the density over all exterior spherical shells:

M = \int_{R}^{\infty} \int_{0}^{π} \int_{0}^{2 π} ρ_{0} e^{- (r - R) / h_{0}} r^{2} sin θ d ϕ d θ d r = 4 π ρ_{0} \int_{0}^{\infty} (R + h)^{2} e^{- h / h_{0}} d h .

Repeated integration by parts, or the substitution

h = h_{0} s

, gives

\int_{0}^{\infty} e^{- h / h_{0}} d h = h_{0}, \int_{0}^{\infty} h e^{- h / h_{0}} d h = h_{0}^{2}, \int_{0}^{\infty} h^{2} e^{- h / h_{0}} d h = 2 h_{0}^{3} .

Expand

(R + h)^{2}

M = 4 π ρ_{0} h_{0} (R^{2} + 2 R h_{0} + 2 h_{0}^{2}) .

For

h_{0} ≪ R

, the leading term is surface area

4 π R^{2}

ρ_{0} h_{0}

; the other terms retain the finite-curvature corrections.

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