Ball.cs

using UnityEngine;
using System.Collections;

public class Ball : MonoBehaviour {
    public float maxHeight;
    public float minHeight;
    public float timeToReachMaxHeight;

    private float jumpVelocity;
    private float gravity;
    private float terminationVelocity;

    private Vector3 pos;
    private bool onPlatform = false;
    private float velocity;

    public void Start() {
        velocity = 0f;

        gravity = (2 * maxHeight) / Mathf.Pow(timeToReachMaxHeight, 2f);
        jumpVelocity = Mathf.Sqrt(2 * gravity * maxHeight);
        terminationVelocity = Mathf.Sqrt(Mathf.Pow(jumpVelocity, 2f) + 2 * gravity * (maxHeight - minHeight));
    }

    public void Update() {
        // Debug.Log(velocity);
        pos = transform.position;

        if(Input.GetKeyDown("space")) {
            Jump();
        }

        pos.y = pos.y + Time.deltaTime * velocity;
        transform.position = pos;

        if(!onPlatform) {
            velocity = velocity - Time.deltaTime * gravity;
        }

        if(velocity > 0) {
            velocity = Mathf.Min(terminationVelocity, velocity);
        }
    }

    public void Jump() {
        velocity = jumpVelocity;
        onPlatform = false;
    }

    public void Stop(float top) {
        pos = transform.position;
        pos.y = top;
        velocity = 0f;
        transform.position = pos;

        onPlatform = true;
    }

}

Platform.cs

using UnityEngine;
using System.Collections;

public class Platform : MonoBehaviour {
    public GameObject ball;
    private float top;


    public void Start() {
        top = transform.position.y;
    }

    void OnTriggerEnter(Collider other) {
        if(other.transform.position.y > top) {
            ball.SendMessage("Stop", top + 1f);
        }
    }

}