Q45 / 54 · JEE Advanced 2020 (Paper 2) · Physics

A rod of mass m and length L, pivoted at one of its ends, is hanging vertically. A bullet of the same mass moving at speed v strikes the rod horizontally at a distance x from its pivoted end and gets embedded in it. The combined system now rotates with an angular speed ω about the pivot. The maximum angular speed ωM is achieved for x=xM. Then

Figure

  • Correct answer
    A
    ω=3vxL2+3x2
  • B
    ω=12vxL2+12x2
  • Correct answer
    C
    xM=L3
  • Correct answer
    D
    ωM=v2L3

Solution

The net torque on the system (rod+bullet) will be zero about hinge point. Therefore, we can apply the principle of angular momentum conservation about hinge point.

From angular momentum conservation,

mvx=mL23+mx2ω

⇒ω=3vxL2+3x2=3vL2x+3x

For maximum angular velocity,

dωdx=0

⇒ddxL2x+3x=0 ⇒-L2x2+3=0 ⇒x=L3

⇒ωmax=3v3L2L+3L=3v2L

Hence, the correct answer is options: 'A', 'C', 'D'.

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