A uniform disc of diameter R/2 is put over another uniform disc of diameter R of same thickness and density. The peripheries of the two discs touch each other. The centre of mass of the system from centre of big disc is

# A uniform disc of diameter R/2 is put over another uniform disc of diameter R of same thickness and density. The peripheries of the two discs touch each other. The centre of mass of the system from centre of big disc is

1. A

R/10

2. B

R/20

3. C

9R/10

4. D

11R/10

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### Solution:

For the disc we know that,

Distance of Cm from big disc =

$\frac{{m}_{2}d}{{m}_{1}+{m}_{2}}=\frac{\left(\frac{{R}^{2}}{16}\right)\left(\frac{R}{4}\right)}{\frac{{R}^{2}}{4}+\frac{{R}^{2}}{16}}=\frac{R}{20}$

Hence the correct answer is R/20.

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