Building trajectory in 2D, getting initial velocity after ForceMode2D.Impulse was applied

Questions & Answers
1

I’m trying to build a simple 2D game where I launch projectile with constant speed and then I have “gravity” areas that pull projectile to them (like gravity around planets or black holes).
I apply those ‘gravity’ forces in the following way in FixedUpdate():

var direction = GetGravityDirection(_projectileCollider);  
_projectileCollider.GetComponent<Rigidbody2D>().AddForce(direction * Force, ForceMode2D.Impulse);

So every ‘black hole’ each fixed update apply some force to my projectile. I launch projectile like this (after I calculate direction and force of launch):

rigidBody2D.AddForce(direction * force, ForceMode2D.Impulse);

Now I want to calculate and build a trajectory. My plan is to calculate it step by step. I know initial position of projectile and force applied to it. I should be able to calculate positions for each fixedDeltaTime. I can’t find the initial velocity. From specs I understood that mode Impulse applies the force for one frame. Therefore I have force, time, mass and initial speed. I should be able to find the velocity of my projectile after first frame (which should be constant afterwards if no other forces are applied to the projectile). I’m trying to find that speed like this:

var gainedVelocity = force * Time.fixedDeltaTime / rigidBody2D.mass; // force is Vector3

None of the variables is 0, but I’m getting zero vector as the result (in fact I’m getting values close to zero). When I launch the projectile it logs some speed.

Can I solve this problem in the way I described? If yes, then what am I doing wrong?
Many thanks.

P.S. I can launch by setting velocity to my projectile, but I still don’t know if the approach I selected will work. Even if I deal with initial velocity, later I need to calculate the impact of each force from ‘black holes’ and I wanted to do this in the same way, by calculating it from F=ma formula and applying it to x1 = x0 + vt + 0.5at*t.

2

I changed starting Force to just velocity for my projectile, but I still have ForceMode2D.Impulse in ‘black holes’. So I have something like this for trajectory:

public void BuildNewPath(Vector3 initialPosition, Vector3 initialVelocity, List<Gravity> affectors, float projectileMass)
{
    _lineRenderer.SetVertexCount(0);
    int stepsCount = 1000;
    float timeDelta = Time.fixedDeltaTime;

    _lineRenderer.SetVertexCount(stepsCount);
    Vector3 position = initialPosition;
    Vector3 velocity = initialVelocity;
    for (int i = 0; i < stepsCount; ++i)
    {
        _lineRenderer.SetPosition(i, position);

        var acceleration = CalculateResultingAcceleration(position, affectors, projectileMass);

        position += velocity * timeDelta + 0.5f * acceleration * timeDelta * timeDelta;
        velocity += acceleration * timeDelta;
    }
}

private Vector3 CalculateResultingAcceleration(Vector3 position, List<Gravity> affectors, float projectileMass)
{
    Vector3 resultingAcceleration = Vector3.zero;
    foreach (Gravity affector in affectors)
    {
        resultingAcceleration += affector.GetForce(position) / projectileMass;
    }
    return resultingAcceleration;
}

But it doesn’t seem to work the way I expected. Should I debug this approach or change ‘black holes’ to change velocity of my projectile in Fixed update instead of applying force? I used forces initially as I wanted to have some kind of non-linear gravity forces, they would change based on distance from projectile to center of each ‘black hole’. I can change this to velocities, but I still don’t know if that is a valid approach to the problem I’m trying to solve.

Thanks