A block of mass m is pressed against a wall by a spring as shown in the figure. The spring has natural length , `l (l gt L)`, and the coefficient of friction between the block and the wall is `mu` . The minimum spring constant K necessary for equilibrium is:

Options

A. (a)`(2 mg)/(mu (l - L))`
B. (b)`(mg)/(mu(l - L))`
C. (c)`(mg)/(2mu (l - L))`
D. (d)`(2 mg)/(mu l)`

Answer

B

Explanation

`N = k (l - L) implies F_("max") = mu k (l - L)`
For equilibrium : `mg le f_("max") implies mg le mu k (l - L)` `implies k ge (mg)/(mu (l - L))`