Unity Ball bouncing code

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

public class gravity : MonoBehaviour {

    public float _gravity;
    public float mass;
    public bool noAirResistance = false;

    private Rigidbody rb;
    private float startingPos;
    private bool grounded = false;
    private float denisty = 1.29f;

    [SerializeField]private float border;
    [SerializeField]private float fallingVelocity;

    private void Start()
    {
        border = 2 * (mass * _gravity) / (0.4f * 1 * 3.14f * denisty);
        startingPos = this.transform.position.y;
        _gravity = (_gravity > 0) ? _gravity : _gravity * (-1);
        rb = this.gameObject.GetComponent<Rigidbody>();
    }

    private void Update()
    {
        if(noAirResistance)
        {
            if (fallingVelocity < 0.5) grounded = false;
            fallingVelocity = calculateFallingVelocity();
            if (!grounded)
                rb.velocity = new Vector3(0, (-1) * fallingVelocity, 0);
            else rb.velocity = new Vector3(0, 1 * fallingVelocity, 0);
            //Debug.Log(fallingVelocity);
        }
        else
        {
            if (!grounded)
            {
                fallingVelocity = calculateFallingVelocity();
                //Debug.Log(fallingVelocity);
                //if (fallingVelocity < border) fallingVelocity = border;
                rb.velocity = new Vector3(0, (-1) * fallingVelocity, 0);
            }
            else
            {
                fallingVelocity = calculateFallingVelocity();

                rb.velocity = new Vector3(0, (1) * fallingVelocity, 0);
                if (fallingVelocity < 1)
                {
                    grounded = false;
                    startingPos = this.transform.position.y;
                }
            }

        }
    }

    private float calculateFallingVelocity()
    {
        float velocity = startingPos + (_gravity * (calculateTime())) / 2;
        return velocity;
    }

    private float calculateTime()
    {
        float time = (2 * (startingPos - this.transform.position.y)) / _gravity;
        Debug.Log(time);
        return time;
    }

    void OnCollisionEnter(Collision c)
    {
        grounded = true;
    }

}