Explain stress versus strain graph with the help of a neat labelled diagram.

 A stone of mass $m$ is tied to an elastic string of negligible mass and spring constant $k$. The unstretched length of the string is $L$ and has negligible mass. The other end of the string is fixed to a nail at a point $P.$ Initially the stone is at the same level as the point $P$. The stone is dropped vertically from point $P.$ Find the distance $y$ from the top when the mass comes to rest for an instant, for the first time.

 A stone of mass $m$ is tied to an elastic string of negligible mass and spring constant $k$. The unstretched length of the string is $L$ and has negligible mass. The other end of the string is fixed to a nail at a point $P.$ Initially the stone is at the same level as the point $P$. The stone is dropped vertically from point $P.$ What shall be the nature of the motion after the stone has reached its lowest point?