Question
A body is executing $$S.H.M.$$ When its displacement from the mean position are $$4\,cm$$ and $$5\,cm,$$ it has velocities $$10\,cm\,{s^{ - 1}}$$ and $$8\,cm\,{s^{ - 1}}$$ respectively. Its periodic time is
A.
$$\frac{\pi }{{2s}}$$
B.
$$\pi \,s$$
C.
$$\frac{{3\,\pi }}{{2s}}$$
D.
$$2\,\pi \,s$$
Answer :
$$\pi \,s$$
Solution :
$$\eqalign{
& {\text{Using}}\,{v^2} = {\omega ^2}\left( {{a^2} - {y^2}} \right)\,{\text{we}}\,{\text{have}} \cr
& {10^2} = {\omega ^2}\left( {{a^2} - {4^2}} \right) \cr
& {\text{and}}\,\,{8^2} = {\omega ^2}\left( {{a^2} - {5^2}} \right); \cr
& {\text{so}}\,\,{10^2} - {8^2} = {\omega ^2}\left( {{5^2} - {4^2}} \right) = \left( {3{\omega ^2}} \right)\,{\text{or}}\,6 = 3\,\omega \cr
& {\text{or}}\,\omega = 2\,{\text{or}}\,T = \frac{{2\,\pi }}{\omega } = \frac{{2\,\pi }}{2} = \pi \,s. \cr} $$