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Rotational Motion question

2023 · Shift 1 · Q38
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Rotational Motion question

2023 · Shift 1 · Q38

JEE AdvancedPhysicsRotational MotionMCQ+3 / −1
A bar of mass M=1.00 kgM=1.00 \mathrm{~kg}M=1.00 kg and length L=0.20 mL=0.20 \mathrm{~m}L=0.20 m is lying on a horizontal frictionless surface. One end of the bar is pivoted at a point about which it is free to rotate. A small mass m=0.10 kgm=0.10 \mathrm{~kg}m=0.10 kg is moving on the same horizontal surface with 5.00 m s−15.00 \mathrm{~m} \mathrm{~s}^{-1}5.00 m s−1 speed on a path perpendicular to the bar. It hits the bar at a distance L/2L / 2L/2 from the pivoted end and returns back on the same path with speed v. After this elastic collision, the bar rotates with an angular velocity ω\omegaω. Which of the following statement is correct?
  1. A
    ω=6.98 rad s−1\omega=6.98 ~\mathrm{rad}~ \mathrm{s}^{-1}ω=6.98 rad s−1 and v=4.30 m s−1\mathrm{v}=4.30 \mathrm{~m} \mathrm{~s}^{-1}v=4.30 m s−1
  2. B
    ω=3.75 rad s−1\omega=3.75 ~\mathrm{rad} ~\mathrm{s}^{-1}ω=3.75 rad s−1 and v=4.30 m s−1\mathrm{v}=4.30 \mathrm{~m} \mathrm{~s}^{-1}v=4.30 m s−1
  3. C
    ω=3.75 rad s−1\omega=3.75 ~\mathrm{rad}~ \mathrm{s}^{-1}ω=3.75 rad s−1 and v=10.0 m s−1\mathrm{v}=10.0 \mathrm{~m} \mathrm{~s}^{-1}v=10.0 m s−1
  4. D
    ω=6.80 rad s−1\omega=6.80 ~\mathrm{rad} ~\mathrm{s}^{-1}ω=6.80 rad s−1 and v=4.10 m s−1\mathrm{v}=4.10 \mathrm{~m} \mathrm{~s}^{-1}v=4.10 m s−1
View written solutionFree

Correct answer: A

  1. Given data

    M=1.00 kg,L=0.20 m,m=0.10 kg,u=5.00 m s−1M=1.00\,\text{kg},\quad L=0.20\,\text{m},\quad m=0.10\,\text{kg},\quad u=5.00\,\text{m s}^{-1}M=1.00kg,L=0.20m,m=0.10kg,u=5.00m s−1

    The particle hits the rod at distance

    r=L2=0.10 mr=\frac{L}{2}=0.10\,\text{m}r=2L​=0.10m

    The rod is pivoted at one end and rotates freely about that pivot.

  2. Moment of inertia of the rod about the pivot

    For a uniform rod about one end,

    I=13ML2=13(1.00)(0.20)2=0.043=0.01333 kg m2I=\frac{1}{3}ML^2=\frac{1}{3}(1.00)(0.20)^2=\frac{0.04}{3}=0.01333\,\text{kg m}^2I=31​ML2=31​(1.00)(0.20)2=30.04​=0.01333kg m2

  3. Use conservation of angular momentum about the pivot

    During collision, external impulse at pivot has zero moment about the pivot, so angular momentum about pivot is conserved.

    Initial angular momentum of the small mass:

    Li=mur=(0.10)(5.00)(0.10)=0.05 kg m2/sL_i=mur=(0.10)(5.00)(0.10)=0.05\,\text{kg m}^2\text{/s}Li​=mur=(0.10)(5.00)(0.10)=0.05kg m2/s

    After collision, the particle returns back along the same path, so its final velocity is opposite in direction. Taking initial direction as positive,

    vf=−vv_f=-vvf​=−v

    Hence final angular momentum of particle is

    Lm,f=−mvrL_{m,f}=-mvrLm,f​=−mvr

    Rod's angular momentum after collision:

    Lrod=IωL_{rod}=I\omegaLrod​=Iω

    Therefore,

    mur=−mvr+Iωmur=-mvr+I\omegamur=−mvr+Iω

    0.05=−0.1×v×0.1+0.01333 ω0.05=-0.1\times v\times 0.1+0.01333\,\omega0.05=−0.1×v×0.1+0.01333ω

    0.05=−0.01v+0.01333ω(1)0.05=-0.01v+0.01333\omega \qquad (1)0.05=−0.01v+0.01333ω(1)

  4. Use conservation of kinetic energy

    Collision is elastic, so total kinetic energy is conserved.

    Initial KE:

    Ki=12mu2=12(0.10)(5.00)2=1.25 JK_i=\frac12 mu^2=\frac12(0.10)(5.00)^2=1.25\,\text{J}Ki​=21​mu2=21​(0.10)(5.00)2=1.25J

    Final KE:

    Kf=12mv2+12Iω2K_f=\frac12 mv^2+\frac12 I\omega^2Kf​=21​mv2+21​Iω2

    So,

    1.25=12(0.10)v2+12(0.01333)ω21.25=\frac12(0.10)v^2+\frac12(0.01333)\omega^21.25=21​(0.10)v2+21​(0.01333)ω2

    1.25=0.05v2+0.006667ω2(2)1.25=0.05v^2+0.006667\omega^2 \qquad (2)1.25=0.05v2+0.006667ω2(2)

  5. Solve equations

    From (1):

    0.01333ω=0.05+0.01v0.01333\omega=0.05+0.01v0.01333ω=0.05+0.01v

    ω=0.05+0.01v0.01333=3.75+0.75v\omega=\frac{0.05+0.01v}{0.01333}=3.75+0.75vω=0.013330.05+0.01v​=3.75+0.75v

    Now test the options.


    Option A: v=4.30v=4.30v=4.30 m/s

    ω=3.75+0.75(4.30)=3.75+3.225=6.975≈6.98 rad/s\omega=3.75+0.75(4.30)=3.75+3.225=6.975\approx 6.98\,\text{rad/s}ω=3.75+0.75(4.30)=3.75+3.225=6.975≈6.98rad/s

    This matches the stated ω\omegaω.

    Check energy:

    Kf=0.05(4.30)2+0.006667(6.975)2K_f=0.05(4.30)^2+0.006667(6.975)^2Kf​=0.05(4.30)2+0.006667(6.975)2

    =0.05(18.49)+0.006667(48.65)=0.05(18.49)+0.006667(48.65)=0.05(18.49)+0.006667(48.65)

    =0.9245+0.3243=1.2488≈1.25 J=0.9245+0.3243=1.2488\approx 1.25\,\text{J}=0.9245+0.3243=1.2488≈1.25J

    So Option A is correct.


    Option B: v=4.30v=4.30v=4.30 gives ω=6.98\omega=6.98ω=6.98, not 3.753.753.75 rad/s. Incorrect.


    Option C: v=10.0v=10.0v=10.0 gives

    ω=3.75+0.75(10)=11.25 rad/s\omega=3.75+0.75(10)=11.25\,\text{rad/s}ω=3.75+0.75(10)=11.25rad/s

    not 3.753.753.75 rad/s. Incorrect.


    Option D: v=4.10v=4.10v=4.10 gives

    ω=3.75+0.75(4.10)=6.825≈6.83 rad/s\omega=3.75+0.75(4.10)=6.825\approx 6.83\,\text{rad/s}ω=3.75+0.75(4.10)=6.825≈6.83rad/s

    close to 6.806.806.80, but now check energy:

    Kf=0.05(4.10)2+0.006667(6.825)2K_f=0.05(4.10)^2+0.006667(6.825)^2Kf​=0.05(4.10)2+0.006667(6.825)2

    =0.8405+0.3107=1.1512 J=0.8405+0.3107=1.1512\,\text{J}=0.8405+0.3107=1.1512J

    which is not equal to 1.251.251.25 J. So incorrect.

  6. Final answer

    The correct option is:

    A: ω=6.98 rad s−1,  v=4.30 m s−1\boxed{\text{A: } \omega=6.98\,\text{rad s}^{-1},\; v=4.30\,\text{m s}^{-1}}A: ω=6.98rad s−1,v=4.30m s−1​

  7. Comparison with stored correct answer

    Stored correct answer: A

    My derived answer also: A

    Hence, they agree.

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