Help me with adding force to instantiated object please!

Unity Engine
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1

I’m new to game development and i can’t seem to get this to work, I’ve been trying for two hours now.

Its supposed to be a proof of concept code but i’m struggling.

The problem is something to do with the adding force to the cloned bullet prefab i have.

I tried using rigid.velocity which is my preferred method but i couldn’t get that to work either.

    private void Update()
    {
        shootingLoc = new Vector2(gunBarrel.position.x, gunBarrel.position.y);
        if (Input.GetMouseButtonDown(0))
        {
            shoot();
        }
    }
    private void shoot()
    {
        GameObject Bullet = Instantiate(pfBullet, shootingLoc, bulletRotation.rotation);
        Rigidbody2D rigid = Bullet.GetComponent<Rigidbody2D>();  
        rigid.AddRelativeForce(Bullet.transform.forward * bulletSpeed);

        Destroy(Bullet, 1f);
    }
2

What is happening? Is the bullet even spawning? Does it just stay in one position? Does it fall to the ground?

A couple things:

  1. Change the 3rd argument in your Instantiate function from bulletRotation.rotation to Quaternion.identity.
  2. Try increasing the bulletSpeed to like 10,000 or something much higher than what you have. (this is what newer folks sometimes don’t realize). If it does move insanely fast, then you know the code works but the force was just too low.
  3. Throw in a Debug.Log() and use it to read the velocity of each object. So after you use AddRelativeForce, on a new line do: Debug.Log(rigid.velocity); Then test the game and check the console window to see what the output is, hopefully it isn’t 0

See what happens and report back

1 Like
3

You also probably want this line:

to just set the velocity, something like

rigid.velocity = Bullet.transform.forward * bulletSpeed;

Way easier than reasoning about forces and masses.

Other than that…

How to report your problem productively in the Unity3D forums:

http://plbm.com/?p=220

This is the bare minimum of information to report:

  • what you want
  • what you tried
  • what you expected to happen
  • what actually happened, log output, variable values, and especially any errors you see
  • links to actual Unity3D documentation you used to cross-check your work (CRITICAL!!!)

The purpose of YOU providing links is to make our job easier, while simultaneously showing us that you actually put effort into the process. If you haven’t put effort into finding the documentation, why should we bother putting effort into replying?

If you post a code snippet, ALWAYS USE CODE TAGS:

How to use code tags: https://discussions.unity.com/t/481379

  • Do not TALK about code without posting it.
  • Do NOT post unformatted code.
  • Do NOT retype code. Use copy/paste properly using code tags.
  • Do NOT post screenshots of code.
  • Do NOT post photographs of code.
  • Do NOT attach entire scripts to your post.
  • ONLY post the relevant code, and then refer to it in your discussion.

If you have no idea what is going on then it is…

Time to start debugging!

By debugging you can find out exactly what your program is doing so you can fix it.

Here is how you can begin your exciting new debugging adventures:

You must find a way to get the information you need in order to reason about what the problem is.

Once you understand what the problem is, you may begin to reason about a solution to the problem.

What is often happening in these cases is one of the following:

  • the code you think is executing is not actually executing at all
  • the code is executing far EARLIER or LATER than you think
  • the code is executing far LESS OFTEN than you think
  • the code is executing far MORE OFTEN than you think
  • the code is executing on another GameObject than you think it is
  • you’re getting an error or warning and you haven’t noticed it in the console window

To help gain more insight into your problem, I recommend liberally sprinkling Debug.Log() statements through your code to display information in realtime.

Doing this should help you answer these types of questions:

  • is this code even running? which parts are running? how often does it run? what order does it run in?
  • what are the names of the GameObjects or Components involved?
  • what are the values of the variables involved? Are they initialized? Are the values reasonable?
  • are you meeting ALL the requirements to receive callbacks such as triggers / colliders (review the documentation)

Knowing this information will help you reason about the behavior you are seeing.

You can also supply a second argument to Debug.Log() and when you click the message, it will highlight the object in scene, such as Debug.Log("Problem!",this);

If your problem would benefit from in-scene or in-game visualization, Debug.DrawRay() or Debug.DrawLine() can help you visualize things like rays (used in raycasting) or distances.

You can also call Debug.Break() to pause the Editor when certain interesting pieces of code run, and then study the scene manually, looking for all the parts, where they are, what scripts are on them, etc.

You can also call GameObject.CreatePrimitive() to emplace debug-marker-ish objects in the scene at runtime.

You could also just display various important quantities in UI Text elements to watch them change as you play the game.

Visit Google for how to see console output from builds. If you are running a mobile device you can also view the console output. Google for how on your particular mobile target, such as this answer for iOS: https://discussions.unity.com/t/700551 or this answer for Android: https://discussions.unity.com/t/699654

If you are working in VR, it might be useful to make your on onscreen log output, or integrate one from the asset store, so you can see what is happening as you operate your software.

Another useful approach is to temporarily strip out everything besides what is necessary to prove your issue. This can simplify and isolate compounding effects of other items in your scene or prefab.

If your problem is with OnCollision-type functions, print the name of what is passed in!

Here’s an example of putting in a laser-focused Debug.Log() and how that can save you a TON of time wallowing around speculating what might be going wrong:

https://discussions.unity.com/t/839300/3

If you are looking for how to attach an actual debugger to Unity: https://docs.unity3d.com/2021.1/Documentation/Manual/ManagedCodeDebugging.html

“When in doubt, print it out!™” - Kurt Dekker (and many others)

Note: the print() function is an alias for Debug.Log() provided by the MonoBehaviour class.

4

Also this ^ ^ ^ ^ if you’re setting it in code on a public or serialized value, it is probably not changing. Here’s why:

Serialized / public fields in Unity are initialized as a cascade of possible values, each subsequent value (if present) overwriting the previous value:

  • what the class constructor makes (either default(T) or else field initializers, eg “what’s in your code”)

  • what may be saved with the prefab

  • what may be saved with the prefab override(s)/variant(s)

  • what may be saved in the scene and not applied to the prefab

  • what may be changed in the scene and not yet saved to disk

  • what may be changed in OnEnable(), Awake(), Start(), or even later

Make sure you only initialize things at ONE of the above levels, or if necessary, at levels that you specifically understand in your use case. Otherwise errors will seem very mysterious.

Here’s the official discussion: https://blog.unity.com/technology/serialization-in-unity

If you must initialize fields, then do so in the void Reset() method, which ONLY runs in the UnityEditor.

Field initializers versus using Reset() function and Unity serialization:

https://discussions.unity.com/t/829681/2

https://discussions.unity.com/t/846251/8

To avoid complexity in your prefabs / scenes, I recommend NEVER using the FormerlySerializedAsAttribute