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Question

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=PfPi
( disc is at rest, Pi=0)
I=mv0
or v=Im
Angular impulse J=ΔL=LfLi ( 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

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