Катание шарика

using UnityEngine;
using System.Collections;
 
public class NewBehaviourScript : MonoBehaviour
{
    private Vector3 inp;
    private Vector3 delta;
    private Vector3 oldPos;
    private Vector3 gravity;
    private const float pi2 = Mathf.PI * Mathf.PI;
 
    public float diam = 7f;
 
    public float speed = 7f;
 
    public Transform cam;
    GameObject fire;
 
    
    // Use this for initialization
        void Start (){
            oldPos = transform.position;
            Physics.gravity = new Vector3(0, -75, 0);
        }
        
        // Update is called once per frame
        void Update ()
{
    fire = GameObject.Find("fire");
     if (Input.GetButtonDown("Jump")){     
    fire.particleEmitter.emit = true;
    diam = 10f;
    speed = 10f;
     }
     if (Input.GetButtonUp("Jump")){ 
fire.particleEmitter.emit = false;
         diam = 4f;
 
    speed = 4f;
     }
            inp.x = Input.GetAxis("Horizontal");
            inp.z = Input.GetAxis("Vertical");
        }
    void FixedUpdate()
        {
            
        Vector3 cp = transform.position - cam.position;
        cp.y = 0f;
        transform.Translate(Quaternion.LookRotation(cp) * inp * speed * Time.deltaTime, Space.World);
        delta = oldPos - transform.position;
 
        float ang = Mathf.Sin(delta.magnitude / diam / pi2) * Mathf.Rad2Deg;
        transform.RotateAround(Vector3.Cross(delta, Vector3.up), ang);
        oldPos = transform.position;
    
}
}