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Question

A ring of mass m is rolling without slipping with linear speed v as shown in figure. Four particles each of mass m are also attached at points A, B, C and D. Find total kinetic energy of the system.


A
6mv2
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B
3mv2
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C
mv2
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D
5mv2
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Solution

The correct option is D 5mv2

Let the radius of the ring be R.

We know that in case of pure rolling,
ω=vR and vA=0

Since the angular velocity ω is constant, so we can write,

ω=vAO=vBAB=vDAD=vCAC...(1)

From,
AO=R ; AB=R2+R2=R2 ; AD=R2+R2=R2 ; AC=2R

From equation 1,

ω=vR=vBR2=vDR2=vC2R

Thus,
vB=vD=2v and vC=2v

Now,
Total kinetic energy =[translational kinetic energy of four particles]+[translational kinetic energy of ring + rotational kinetic energy of ring]

Substituting the values, we get
(KE)total=[12m(0)2+12m(2v)2+12m(2v)2+12m(2v)2]+[12mv2+12Iω2]

Substituing I=mR2 and ω=vR, we get
(KE)total=[0+m(v)2+m(v)2+2m(v)2]+[12mv2+12×mR2(vR)2]

(KE)total=5mv2

Hence, option (d) is the correct answer.

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