ml-agents/unity-environment/Assets/ML-Agents/Examples/Tennis/Scripts/TennisArea.cs

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

public class TennisArea : MonoBehaviour {

    public GameObject ball;
    public GameObject agentA;
    public GameObject agentB;

	// Use this for initialization
	void Start () {
		
	}
	
	// Update is called once per frame
	void Update () {
		
	}

    public void MatchReset() {
        float ballOut = Random.Range(4f, 11f);
        int flip = Random.Range(0, 2);
        if (flip == 0)
        {
            ball.transform.position = new Vector3(-ballOut, 5f, 0f) + transform.position;
        }
        else
        {
            ball.transform.position = new Vector3(ballOut, 5f, 0f) + transform.position;
        }
        ball.GetComponent<Rigidbody>().velocity = new Vector3(0f, 0f, 0f);
        ball.transform.localScale = new Vector3(1, 1, 1);
    }

    void FixedUpdate() {
        Vector3 rgV = ball.GetComponent<Rigidbody>().velocity;
        ball.GetComponent<Rigidbody>().velocity = new Vector3(Mathf.Clamp(rgV.x, -9f, 9f), Mathf.Clamp(rgV.y, -9f, 9f), rgV.z);
    }
}