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QJEEM23Ph358e: JEE Main 2023 Physics - Class 11 JEEM23Ph358e Solution: A simple pendulum with length and bob of mass is executing…

A simple pendulum with length 100 cm100 \mathrm{~cm} and bob of mass 250 g250 \mathrm{~g} is executing S.H.M. of amplitude 10 cm10 \mathrm{~cm} . The maximum tension in the string is found to be x40 N\frac{x}{40} \mathrm{~N} . The value of xx is ___________.

Step-by-Step Verified Solution

Given the amplitude AA and the length ll of the pendulum, we can find the maximum angular displacement θ0\theta_0 :

sin⁡θ0=Al=10100=110\sin \theta_0 = \frac{A}{l} = \frac{10}{100} = \frac{1}{10}

By conservation of energy, the following equation holds:

12mv2=mgl(1−cos⁡θ)\frac{1}{2} m v^2 = m g l(1 - \cos \theta)

The maximum tension occurs at the mean position (i.e., when the pendulum is vertical). At this point, we have:

amp;T−mg=mv2l&⇒T=mg+mv2l\begin{aligned}& T - mg = \frac{m v^2}{l} \& \Rightarrow T = mg + \frac{m v^2}{l}\end{aligned}

Substituting the conservation of energy equation, we get:

amp;T=mg+2mg(1−cos⁡θ)amp;=mg[1+2(1−1−sin⁡2θ)]amp;=mg[3−21−1100]amp;=2501000×10[3−2(1−1200)]=10140amp;∴x=101\begin{aligned}& T = mg + 2 m g(1 - \cos \theta) \\\\& = mg\left[1 + 2\left(1 - \sqrt{1 - \sin^2 \theta}\right)\right] \\\\& = mg\left[3 - 2 \sqrt{1 - \frac{1}{100}}\right] \\\\& = \frac{250}{1000} \times 10\left[3 - 2\left(1 - \frac{1}{200}\right)\right] = \frac{101}{40} \\\\& \therefore x = 101\end{aligned}

So, the value of xx is 101.

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