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Question

Two discs are rotating about the same axis passing through their centres and perpendicular to both discs' planes. If disc 1 has mass 5 kg and 30 cm radius and disc 2 has mass 7 kg and 60 cm radius, then find the moment of intertia about an axis OO'.


A
1.26 kg-m2
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B
1.96 kg-m2
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C
1.48 kg-m2
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D
1.16 kg-m2
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Solution

The correct option is C 1.48 kg-m2
Given, mass of disc 1, m1= 5 kg
& radius of disc 1, r1= 30 cm
Mass of disc 2, m1= 7 kg
& radius of disc 2, r2 = 60 cm

MOI of disc 1 about an axis OO' ,
I1=m1r212=5×(0.30)22=0.225 kg-m2
MOI of disc 2 about an axis OO'' ,
I2=m2r222=7×(0.60)22=1.26 kg-m2

I=I1+I2=0.225+1.26=1.48 kg -m2
I=1.48 kg-m2

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