2 A particle \(P\) of mass \(m\) is attached to one end of a light elastic spring of natural length \(a\) and modulus of elasticity 5 mg . The other end of the spring is attached to a fixed point \(O\). The spring hangs vertically with \(P\) below \(O\). The particle \(P\) is pulled down vertically and released from rest when the length of the spring is \(\frac { 3 } { 2 } a\). Find the distance of \(P\) below \(O\) when \(P\) first comes to instantaneous rest.