Q: An ideal spring with spring-constant k is hung from the ceiling and a block of mass M is attached to its lower end. The mass is released with the spring initially unstretched. Then the maximum extension in the spring is

(a)$\displaystyle \frac{4 M g}{k}$

(b) $\displaystyle \frac{2 M g}{k}$

(c) $\displaystyle \frac{ M g}{k}$

(d) $\displaystyle \frac{ M g}{2 k}$

Ans: (b)

$\displaystyle M g x = \frac{1}{2} k x^2 $

$\displaystyle x = \frac{2 M g}{k}$