Question

Two circular discs of moment of inertia $0.2kg{m}^2$ and $0.3kg{m}^2$ are rotating about their own axis at the rate of $200rpm$ and $100rpm$ both in clockwise order. If both are gently brought in contact to rotate about same axis, the angular velocity of combination is:

• A. $120rpm$
• B. $140rpm$
• C. $150rpm$
• D. $175rpm$

Answer 1

Answer: (B)

$140rpm$

Final MI $I=0.2+0.3=0.5kg{m}^2$ and the will have a common angular velocity $\omega$ and rpm $N$

Using angular momentum conservation ,

We have $0.2\times 200+0.3\times 100=0.5\times N$

So $N=140rpm$