A uniform disc of mass m and radius R is placed on a smooth horizontal surface. An impulse I is given horizontally to the sphere at height h=3R5 above the centre line. Then which of the following option(s) is correct?
A
The minimum time after which the highest point touches the ground is πRm2I
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B
The minimum time after which the highest point touches the ground is 5πRm6I
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C
The displacement of the COM during this interval is 5πR6
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D
The displacement of the COM during this interval is πR2
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Solution
The correct options are B The minimum time after which the highest point touches the ground is 5πRm6I C The displacement of the COM during this interval is 5πR6 Assume just after impulse given to the disc, linear velocity of COM of the disc is v and angular velocity is ω.
Impulse I=ΔP=Pf−Pi (∵disc is at rest,Pi=0) I=mv−0 or v=Im Angular impulse J=ΔL=Lf−Li(∵disc is at rest initially,Li=0) I×h=Lf I×(3R5)=I0ω Here, I0=mR22 I×(3R5)=(mR22).ω or ω=6I5Rm
When the highest point reaches the ground, it rotates by an angle π. Hence θ=ωt π=(6I5Rm)t t=5πRm6I And the distance travelled by the COM in that time d=v.t d=Im.(5πRm6I) d=5πR6