A uniform rod of length `L` and mass `m` is free to rotate about a frictionless pivot at one end as shown in figure. The rod is held at rest in the horizontal position and a coin of mass `m` is placed at the free end. Now the rod is released The reaction on the coin immediately after the rod starts falling is

Options

A. `(3mg)/(2)`
B. `2mg`
C. zero
D. `(mg)/(2)`

Answer

C

Explanation

`alpha=(tau_("net"))/(I)`
`=((mg((l)/(2))+mgl)/(ml^(2)//3))`
`=(4.5g)/(l)`
`a_("coin")=lalpha=4.5g`
`mg-N=ma_("coin")`
`thereforeN=mg-ma_("coin")=-3.5g`

Since, N can't be negative.
So, `N=0` and coin can not remain in contact with rod.