A small coin of mass `80g` is placed on the horizontal surface of a rotating disc. The disc starts from rest and is given a constant angular acceleration `alpha=2rad//s^(2)`. The coefficient of static friction between the coin and the disc is `mu_(s)=3//4` and cofficient of kinetic friction is `mu_(k)=0.5`. The coin is placed at a distance `r=1m` from the centre of the disc. The magnitude of the resultant force on the coin exerted by the disc just before it starts slipping on the disc is

Options

A. `0.2 N`
B. `0.3` N
C. `0.4`N
D. 1N

Answer

D

Explanation

Resultant reaction by disc to the coin = `sqrt(N^(2) + f^(2))`
N = mg , f = `mu_(s)` mg = mg `sqrt(1 + mu_(s)^(2))` = 1N