Catapult press spacebar to launch / throw

hi everyone. i want to ask how to make the catapult throwing an object by pressing space (Not instantiate) ? It’s similar with Angry bird game (pull the slingshot and release it, then the object will fly how far according to how far you pull the slingshot). You understand what i mean right ? sorry for my bad english.

any answers appreciated.

As I wanted to try for myself as well I came up with a simple form of it.
The object in this case is only going up and falling down at a rate that is not the gravity. You still need to perfect it but at least you got a lead. What would you learn if I just give you the answer?

var Start:float;    
var End:float;
var force:float = 0.0f;

function Update(){
      // when space is pressed down the counter starts
      if (Input.GetKeyDown (KeyCode.Space)){
			Start = Time.time;
	}
    // When space is released the counter stops note that GetKeyDown/GetKeyUp does not do anything while keeping pressed
	if (Input.GetKeyUp (KeyCode.Space)){
			End = Time.time;
			force = (End-Start)*20;  // here you compare the 2 values I had to multiply by 20 to get it up (that's what she says) but you might have to alter this eqution to get what you want		
	}
	// the result is applied to the object rigidbody, only up in this case here again yo need to work it out
	rigidbody.AddForce(Vector3.up*force);
	force = force*0.9;  // force is decreased in time to get the object falling down
	Debug.Log("Start: "+Start+" End: "+End+" Force: "+force);//Just for debug...
}

Hope that helps.