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?