Problem 13.19

T

พิจารณาแผนภาพอิสระของวัตถุ

\(\theta\)

\(\frac{kq^{2}}{(2\ell \sin \theta )^{2}}m\)

จาก \(\Sigma F_{x}= 0\)

mg

\(Tsin\theta -\frac{kq^{2}}{(2\ell \sin \theta )^{2}}= 0; r = 2\ell \sin \theta\)

\(Tsin\theta =\frac{kq^{2}}{4\ell ^{2}\sin ^{2}\theta}\cdots \cdots (1)\)

จาก \(\Sigma F_{y}= 0\)

\(Tcos\theta - mg = 0\)

\(Tcos\theta = mg\cdots \cdots (2)\)

\((1)/(2) ; \tan \theta =\frac{kq^{2}}{4mg\ell ^{2}\sin ^{2}\theta}\)

\(q = 2\ell \sin \theta \sqrt{\frac{mgtan\theta}{k}}\;\blacksquare\)