How to do it...

1. Create the Projectile class, along with its member variables, to handle the physics:

using UnityEngine; 
using System.Collections; 
 
public class Projectile : MonoBehaviour 
{ 
    private bool set = false; 
    private Vector3 firePos; 
    private Vector3 direction; 
    private float speed; 
    private float timeElapsed; 
} 

2. Define the Update function:

void Update () 
{ 
    if (!set) 
        return; 
    timeElapsed += Time.deltaTime; 
    transform.position = firePos + direction * speed * timeElapsed; 
    transform.position += Physics.gravity * (timeElapsed * timeElapsed) / 2.0f; 
    // extra validation for cleaning the scene 
    if (transform.position.y < -1.0f) 
        Destroy(this.gameObject);// or set = false; and hide it 
} 

3. Finally, implement the Set function in order to fire the game object (for example, calling it after it is instantiated in the scene):

public void Set (Vector3 firePos, Vector3 direction, float speed) 
{ 
    this.firePos = firePos; 
    this.direction = direction.normalized; 
    this.speed = speed; 
    transform.position = firePos; 
    set = true; 
}