A person of mass M is sitting on a swing of length L and swinging with and an angular amplitude θ 0 .…

A person of mass M is sitting on a swing of length L and swinging with and an angular amplitude θ0. If the person stands up when the swing passes through its lowest point, the work done by him, assuming that his centre of mass moves by a distance l l<<L, is close to:
  1. Mgl (1-θ02)
  2. Mgl(1+θ022)
  3. Mgl
  4. Mgl(1+θ02)

Solution

From angular momentum conservation
MV0L=MV(L-l)V=V0LL-l
Now
W=Mgl+12MV02LL-l2-V02
Since lL
W=Mgl+12MV021-lL-2-V02=Mgl+12MV021+2lL-V02=Mgl+MV02lL
From SHM
V0=ωA=gL×Lθ0=gLθ0
Using this
W=Mgl+MgL×θ02×lLW=Mgl1+θ02

Asked in: JEE Main 2019 (12 Apr Shift 1)

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