question_answer

A composite disc is to be made using equal masses of aluminium and iron so that it has as high a moment of inertia as possible. This is possible when 

A)  the surfaces of the discs are made of iron with aluminium inside

B)  the whole of aluminium is kept in the core and the iron at the outer rim of the disc

C)  the whole of the iron is kept in the core and the aluminium at the outer rim of the disc

D)  the whole disc is made with thin alternate sheets of iron and aluminium

Correct Answer: B

Solution :

 We know density of iron > density of aluminium Moment of inertia \[I=I=\int_{{}}^{{}}{{{r}^{2}}\,dm.}\] \[\therefore \]Since, \[{{\rho }_{iron}}>{{\rho }_{alu\min ium}}\] Hence to get high a moment of inertia whole of aluminium is kept in the core and the iron at the outer rim of the disc.