How to make object fall and cause explosion when it hit onto Terrain

i have a sphere object stay at a height of 5 above the plane. under my HUB script,i have some script below.So if the Timer>100. how am i suppose to make the object(sphere) fall and create explosion when it hit the terrain.can some1 help me or gave me some information thx alot.

var timer : float = 0;

function OnGUI () 
    if(timer > 100)
       GUI.Label( Rect( 5, 70, 100, 100 ), FuelGauge[10] );
       Debug.Log("100 sec passed - No Fuel");

function Update () 
    timer = Time.time * 1;

You could probably treat your sphere as a kinematic rigidbody and turn off kinematic when the timer condition is met. Then gravity would make sure it falls to the ground. Finally, you can detect if you collide with the ground with OnCollisionEnter and if you do, spawn your explosion prefab.

i have create a sphere and attached with this script below.can any1 tell me how to conbine this script with the top timer script.,so basically i wanted to use the timer script to tell my sphere do somethings. As in if(timer>100) i want my this sphere to fall and explosion, and i have no idea how to do it.can some1 edit for me thx alots

var explosion : Transform;

function OnCollisionEnter(){




     var explosion1 : Transform;


     explosion1 = Instantiate(explosion,transform.position,transform.rotation);