How to increase rate of a falling object in pooling object?

i m currently working on a script where i want to drop random objects from a list of prefabs. i am using pool objects for this. The problem is as follow: i want these objects one by one when game starts. The rate of falling objects should be slow when game starts. But as time passes, the objects shold fall with increasing more speed. How to do that?

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

public class Poolingscript2 : MonoBehaviour {

public	GameObject[] allprefabs;   //array of prefab
public	List<GameObject> bullets ; 
public	int listsize;
GameObject go;
// Use this for initialization
void Start ()
{
	createlist ();

}

void createlist() //creates list of gameobjects from array of prefabs
{
 	allprefabs = Resources.LoadAll<GameObject> ("Prefab"); //reload prefabs in array

	for (int i=0; i<listsize; i++) 
	{
		foreach (GameObject allprifab in allprefabs) 
		{
			go = Instantiate(allprifab)as GameObject; //make object from aray
			go.SetActive(false);
			bullets.Add (go); //add to list

		}

	}

}

GameObject temp;
void fallobject()
{
	int randm = Random.Range(0,listsize); //select random number
	temp = bullets.ElementAt(randm);      // assign number and select random object
	bullets.Remove(temp);   // remove that random object
	temp.transform.position = transform.position; 
	temp.SetActive(true);   //activate, so it falls down
	Debug.Log ("removed element no: " +randm);	

}
public void Update()
{
		
}

public void addtolist(GameObject g)  //function for adding fallen object back to list
{
	g.SetActive (true);
	bullets.Add (g);

}

}

Every time one drops, increase the mass by a certain amount.

temp.GetComponent.<Rigidbody>().mass = newMass

and every time one falls, increase newMass variable by, say, 0.1