5 A particle \(P\) of mass 0.2 kg is attached to one end of a light elastic string of natural length 0.75 m and modulus of elasticity 21 N . The other end of the string is attached to a fixed point \(A\) which is 0.8 m vertically above a smooth horizontal surface. \(P\) rests in equilibrium on the surface.

1. Find the magnitude of the force exerted on \(P\) by the surface.
   \(P\) is now projected horizontally along the surface with speed \(3 \mathrm {~ms} ^ { - 1 }\).
2. Calculate the extension of the string at the instant when \(P\) leaves the surface.
3. Hence find the speed of \(P\) at the instant when it leaves the surface.