5 One end of a light elastic string, of natural length 0.5 m and modulus of elasticity 140 N , is attached to a fixed point \(O\). A particle of mass 0.8 kg is attached to the other end of the string. The particle is released from rest at \(O\). By considering the energy of the system, find

1. the speed of the particle when the extension of the string is 0.1 m ,
2. the extension of the string when the particle is at its lowest point.