Tower Defense: Calculate Force of projectile depending on Enemy position

I'm going to cheapskate this a little bit and refer you to a previous answer I've given. Your diagram and post don't explicitly state whether you're referring to a 2D or 3D game, so I'll simply offer two relevant links:

Here is my response to a question regarding 2D trajectory: http://answers.unity3d.com/questions/1124197/how-can-i-launch-a-gameobject-at-a-target-if-i-am.html

That also links to my main post relating to 3D trajectory: http://answers.unity3d.com/questions/1087568/3d-trajectory-prediction.html#answer-1087707

By your description, you would be able to base it on my second (3D) example of:

A -> B, attempting to arrive based on a specified launch angle

I use this, found it myself a while back, but works for most cases where you need to hit a moving target from a static one.

Rather then calculating the speed the round needs to go it calculates a new aim point given the targets speed, distance and the known speed the round will travel. if you adjust the speed of the round then it will be more unrealistic then not using gravity. You will have to test a bit to get the correct speed for your rounds, to simulate the effects you want, but then run through his and you should get what your after.

It does add a 'tilt' to the projectile, but you can adjust the spawn point to avoid it spawning within the weapon itself. And of cause this only works to launch the round at the target. if the target seeds up or changes direction the round will miss.

use it like this

  void Update(){
      transform.LookAt(CalculateLead(target, weapon.transform,  speed))                 
      transform.forward = GetComponent<Rigidbody>().velocity;
     } 
 
     
     
     public static Vector3 CalculateLead(GameObject target, Transform t, float speed)
         {
             Vector3 V = target.GetComponent<Rigidbody>().velocity;
             Vector3 D = target.transform.position - t.position;
             float A = V.sqrMagnitude - (speed * speed);
             float B = 2 * Vector3.Dot(D, V);
             float C = D.sqrMagnitude;
             if (A >= 0)
             {
                 Debug.LogError("No solution exists");
                 return target.transform.position;
             }
             else {
                 float rt = Mathf.Sqrt(B * B - 4 * A * C);
                 float dt1 = (-B + rt) / (2 * A);
                 float dt2 = (-B - rt) / (2 * A);
                 float dt = (dt1 < 0 ? dt2 : dt1);
                 return target.transform.position + (V * dt);
             }
         }
 

Unity does have a good way to calculate such physics. Presuming that you are new to Unity, Unity has a neat script called a 'Collider'. Go into your project, locate the inspector, click 'Add Component', and then search 'collider' and select your desired shape for your collider (there're tons!).

Add a collider script to both your arrow prefab and your enemy prefab.

Assuming that you want your arrow to be affected by physics, add the 'Rigidbody' script to it.

To test it's velocity, you must use the OnCollisionEnter function coupled with some code, it will look somewhat like this;

 void OnCollisionEnter(Collision collision) {
         if (collision.relativeVelocity.sqrMagnitude > 2)
             //Executes code if squared velocity is more than 2
     }
 

Here, you are using the OnCollosionEnter function (broadcasted once your collider hits something) which returns a Collision (in this case, we name it collision). Then we check the collision's relative velocity (a Vector 3) and then we simply get it's squared Magnitude (the sqrt function is quite memory-expensive) and compare it to two or whatever you like. For me, I just play around with the limit variable until I find out it's perfect.

Hopefully that helps and good luck with your game ;)