issues with collision detection

I’m currently making a gravity system, and the gravity works great, but the problem is that my collision with my Ground object isn’t working at all.
Here’s the code i have:
using UnityEngine;
using System.Collections;

public class Gravity : MonoBehaviour
{
	public float playerGravity = 5;

	void Update () 
	{
		transform.Translate(Vector3.down * playerGravity * Time.deltaTime);
	}

	void OnCollisionEnter (Collision col)
	{
		if(col.gameObject.name == "Ground")
		{
			playerGravity = 0;
		}
	}
}

whats wrong with my OnCollisionEnter script?

I think your game
object needs a rigidbody component for collision to work.