Question
A particle performs simple harmonic mition with amplitude $$A.$$ Its speed is trebled at the instant that it is at a distance $$\frac{{2A}}{3}$$ from equilibrium position. The new amplitude of the motion is
A.
$$A\sqrt 3 $$
B.
$$\frac{{7A}}{3}$$
C.
$$\frac{A}{3}\sqrt {41} $$
D.
$$3A$$
Answer :
$$\frac{{7A}}{3}$$
Solution :
We know that $$V = \root \omega \of {{A^2} - {x^2}} $$
Initially $$V = \root \omega \of {{A^2} - {{\left( {\frac{{2A}}{3}} \right)}^2}} $$
Finally $$3v = \root \omega \of {A{'^2} - {{\left( {\frac{{2A}}{3}} \right)}^2}} $$
Where $$A' = $$ final amplitude (Given at $$x = \frac{{2A}}{3},$$ velocity to trebled)
On dividing we get $$\frac{3}{1} = \frac{{\sqrt {A{'^2} - {{\left( {\frac{{2A}}{3}} \right)}^2}} }}{{\sqrt {{A^2} - {{\left( {\frac{{2A}}{3}} \right)}^2}} }}$$
$$9\left[ {{A^2} - \frac{{4{A^2}}}{9}} \right] = A{'^2} - \frac{{4{A^2}}}{9}\,\,\,\,\,\,\,\,\,\,\,\therefore A' = \frac{{7A}}{3}$$