Verified Solution • 2025–26 Session moderate

QJEEM19Ph8cd9: JEE Main 2019 Physics - Class 11 JEEM19Ph8cd9 Solution: A metal coin of mass 5 g and radius 1 cm is fixed to a thin…

A metal coin of mass 5 g and radius 1 cm is fixed to a thin stick AB of negligible mass as shown in the figure. The system is initially at rest. The constant torque, that will make the system rotate about AB at 25 rotations per second in 5s, is close to : JEE Main 2019 (Online) 10th April Evening Slot Physics - Rotational Motion Question 150 English

Multiple Choice Options

  • A 7.9 × 10–6 Nm
  • B 4.0 × 10–6 Nm
  • C 2.0 × 10–5 Nm
  • D 1.6 × 10–5 Nm

Step-by-Step Verified Solution

m = 5 × 10–3 kg, r = 10–2 m
ω\omega = 25 × 2 π\pi rad/5
= 50 ω\omega rad/sec

ω=τIt\omega = {\tau \over I}t
τ=Iωt=5mr24×ωt\tau = {{I\omega } \over t} = {{5m{r^2}} \over 4} \times {\omega \over t}

=5×5×10−3×10−4×50π4×5= {{5 \times 5 \times {{10}^{ - 3}} \times {{10}^{ - 4}} \times 50\pi } \over {4 \times 5}}

=25π4×10−6=2×10−5= {{25\pi } \over 4} \times {10^{ - 6}} = 2 \times {10^{ - 5}} Nm

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