Question Number 84231 by Rio Michael last updated on 10/Mar/20 | ||
$$\mathrm{A}\:\mathrm{compound}\:\mathrm{pendulum}\:\mathrm{oscillates}\:\mathrm{though}\:\mathrm{a} \\ $$ $$\mathrm{small}\:\mathrm{angle}\:\theta\:\mathrm{about}\:\mathrm{its}\:\mathrm{equilibrium}\:\mathrm{position} \\ $$ $$\mathrm{such}\:\mathrm{that} \\ $$ $$\:\:\:\mathrm{10}{a}\left(\frac{{d}\theta}{{dt}}\right)^{\mathrm{2}} \:=\:\mathrm{4}{g}\:\mathrm{cos}\:\theta\:,\:{a}\:>\mathrm{0}\:.\:\mathrm{its}\:\mathrm{period}\:\mathrm{is}\: \\ $$ $$\left[\mathrm{A}\right]\:\mathrm{2}\pi\sqrt{\frac{\mathrm{5}{a}}{\mathrm{4}{g}}\:\:}\:\:\:\left[\mathrm{B}\right]\:\frac{\mathrm{2}\pi}{\mathrm{5}}\sqrt{\frac{{a}}{{g}}}\:\:\left[\mathrm{C}\right]\:\mathrm{2}\pi\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}\:}\:\:\left[\mathrm{D}\right]\:\mathrm{2}\pi\sqrt{\frac{\mathrm{5}{a}}{{g}}}\: \\ $$ | ||
Answered by TANMAY PANACEA last updated on 10/Mar/20 | ||
$$\frac{{d}\theta}{{dt}}={w}=\frac{\mathrm{2}\pi}{{T}}\rightarrow{T}=\frac{\mathrm{2}\pi}{{w}} \\ $$ $${w}^{\mathrm{2}} =\left(\frac{{d}\theta}{{dt}}\right)^{\mathrm{2}} =\frac{\mathrm{4}{g}}{\mathrm{10}{a}}{cos}\theta\approx\frac{\mathrm{4}{g}}{\mathrm{10}{a}}×\mathrm{1} \\ $$ $${w}=\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}}\:\rightarrow{T}=\frac{\mathrm{2}\pi}{\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}}} \\ $$ $${T}=\mathrm{2}\pi×\sqrt{\frac{\mathrm{5}{a}}{\mathrm{2}{g}}}\:\:\:\rightarrow\:{i}\:{think}\:{so}\: \\ $$ | ||
Commented byRio Michael last updated on 10/Mar/20 | ||
$$\mathrm{thanks},{i}'\mathrm{ve}\:\mathrm{gotten}\:\mathrm{the}\:\mathrm{idea} \\ $$ | ||
Commented bymr W last updated on 11/Mar/20 | ||
$${answer}\:{is}\:{wrong}\:{sir}! \\ $$ $$\omega\:{here}\:{is}\:{not}\:{the}\:{angular}\:{velocity},\:{i}.{e}. \\ $$ $$\omega\neq\frac{{d}\theta}{{dt}} \\ $$ $$\omega\:{is}\:{the}\:{angular}\:{frequency}. \\ $$ $${we}\:{know}\:{the}\:{angular}\:{velocity}\:\frac{{d}\theta}{{dt}}\:{is} \\ $$ $${not}\:{constant},\:{otherwise}\:{there}\:{is}\:{no} \\ $$ $${oscillation}!\:{but}\:\omega\:{is}\:{a}\:{constant}! \\ $$ $${in}\:{fact}\:\omega\:{here}\:{is}\:{defined}\:{as}\:\sqrt{\frac{{elasticity}}{{mass}}}. \\ $$ $${please}\:{refer}\:{to}\:{your}\:{physics}\:{book}. \\ $$ $$ \\ $$ $${correct}\:{answer}: \\ $$ $$\mathrm{10}{a}\left(\frac{{d}\theta}{{dt}}\right)^{\mathrm{2}} =\mathrm{4}{g}\:\mathrm{cos}\:\theta \\ $$ $$\Rightarrow\frac{{d}\theta}{{dt}}=\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}}×\sqrt{\mathrm{cos}\:\theta} \\ $$ $$\Rightarrow\frac{{d}^{\mathrm{2}} \theta}{{dt}^{\mathrm{2}} }=\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}}×\frac{−\mathrm{sin}\:\theta}{\mathrm{2}\sqrt{\mathrm{cos}\:\theta}}×\frac{{d}\theta}{{dt}} \\ $$ $$\Rightarrow\frac{{d}^{\mathrm{2}} \theta}{{dt}^{\mathrm{2}} }=\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}}×\frac{−\mathrm{sin}\:\theta}{\mathrm{2}\sqrt{\mathrm{cos}\:\theta}}×\sqrt{\frac{\mathrm{2}{g}}{\mathrm{5}{a}}}×\sqrt{\mathrm{cos}\:\theta} \\ $$ $$\Rightarrow\frac{{d}^{\mathrm{2}} \theta}{{dt}^{\mathrm{2}} }=−\frac{{g}\:\mathrm{sin}\:\theta}{\mathrm{5}{a}}\approx−\frac{{g}}{\mathrm{5}{a}}\theta \\ $$ $$\Rightarrow\frac{{d}^{\mathrm{2}} \theta}{{dt}^{\mathrm{2}} }+\frac{{g}}{\mathrm{5}{a}}\theta=\mathrm{0} \\ $$ $$\Rightarrow\frac{{d}^{\mathrm{2}} \theta}{{dt}^{\mathrm{2}} }+\omega^{\mathrm{2}} \theta=\mathrm{0}\:{with}\:\omega=\sqrt{\frac{{g}}{\mathrm{5}{a}}} \\ $$ $${period}\:{T}=\frac{\mathrm{2}\pi}{\omega}=\mathrm{2}\pi\sqrt{\frac{\mathrm{5}{a}}{{g}}} \\ $$ $$\Rightarrow{answer}\:\left[{D}\right]\:{is}\:{correct}. \\ $$ | ||
Commented byTANMAY PANACEA last updated on 11/Mar/20 | ||
$${yes}\:{sir}\:{you}\:{are}\:{right}...{so}\:{pls}\:{ignore}\:{my}\:{post} \\ $$ | ||