5 A particle \(P\) of mass 0.28 kg is attached to the mid-point of a light elastic string of natural length 4 m . The ends of the string are attached to fixed points \(A\) and \(B\) which are at the same horizontal level and 4.8 m apart. \(P\) is released from rest at the mid-point of \(A B\). In the subsequent motion, the acceleration of \(P\) is zero when \(P\) is at a distance 0.7 m below \(A B\).

1. Show that the modulus of elasticity of the string is 20 N .
2. Calculate the maximum speed of \(P\).