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JEE Advanced 2022 Paper 2 · Physics · Kinematics and Laws of Motion · hard

A small block rests on a horizontal turntable at a distance r=1.0 m from its axis; the coefficient of static friction between block and table is 0.5.…

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A small block rests on a horizontal turntable at a distance r=1.0 mr=1.0\ \mathrm m from its axis; the coefficient of static friction between block and table is 0.50.5. The turntable is started from rest and speeds up with constant angular acceleration α=3 rad s−2\alpha=3\ \mathrm{rad\,s^{-2}}. Take g=10 m s−2g=10\ \mathrm{m\,s^{-2}}. Which of the following is/are correct?
  1. A. The block starts to slip at t=23 st=\dfrac23\ \mathrm s.
  2. B. At the instant just before slipping, the friction force on the block makes an angle of about 37∘37^\circ with the line joining the block to the axis.
  3. C. The block slips only after the turntable has completed at least one full revolution.
  4. D. The speed of the block just before it starts to slip is 2 m s−12\ \mathrm{m\,s^{-1}}.
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Answer: A, B, D

The block has tangential acceleration at=αr=3 m s−2a_t=\alpha r=3\ \mathrm{m\,s^{-2}} and centripetal acceleration ac=ω2ra_c=\omega^2r, both supplied by static friction, so f/m=at2+ac2f/m=\sqrt{a_t^2+a_c^2}. Slipping begins when this equals μg=5 m s−2\mu g=5\ \mathrm{m\,s^{-2}}: ac=25−9=4a_c=\sqrt{25-9}=4, hence ω2=4\omega^2=4, ω=2 rad s−1\omega=2\ \mathrm{rad\,s^{-1}}. Then t=ω/α=23 st=\omega/\alpha=\dfrac23\ \mathrm s: (A) is correct. The friction components are 33 (tangential) and 44 (towards the axis), so the angle with the radial line satisfies tan⁡ϕ=3/4\tan\phi=3/4, ϕ≈37∘\phi\approx37^\circ: (B) is correct. The angle turned is θ=ω22α=46=23 rad\theta=\dfrac{\omega^2}{2\alpha}=\dfrac46=\dfrac23\ \mathrm{rad}, which is only about 0.10.1 revolution, so (C) is incorrect. The speed is v=ωr=2 m s−1v=\omega r=2\ \mathrm{m\,s^{-1}}: (D) is correct.
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