Problem 19.36

จาก \(Σ\vec{F} = 0\)

\(F_{Laser}- F_{g}= 0\)

\(F_{Laser}= F_{g}\)

\(PA= mg\)

\((\frac{(f + 1)I}{c})(\pi r^{2}) = mg\)

\((\frac{(0 + 1)I}{c})(\pi r^{2}) = mg; f = 0\)

\(I =\frac{mgc}{\pi r^{2}}\)

จาก \(ρ = \frac{m}{V}\)

\(V =\frac{m}{ρ}\)

\(\pi r^{3} =\frac{4}{3}\frac{m}{ρ}\)

\(_{1/3}\)

\(r = (\frac{3m}{4\pi ρ})\)

\(_{2/3}\)

\(\therefore I =\frac{mgc}{\pi}(\frac{4\pi ρ}{3m})\)

\(_{2/3}\)

\(I = gc(\frac{4ρ m}{3 \pi})\;\blacksquare\)