When is Time.fixedDeltaTime used in fixedUpdate?

You almost never have to use Time.fixedDeltaTime specifically .

That's because Time.deltaTime is a getter property, and if you use it in any method that gets called during the physics step (FixedUpdate, OnCollision/TriggerEnter/Exit/Stay, coroutines resuming after a WaitForFixedUpdate etc), it automatically returns Time.fixedDeltaTime anyway. So it suffices to use Time.deltaTime whenever you want a time delta, and the right one will be selected for you automatically, based on what part of the update loop is running your code.

So the real question is: when should you multiply by a time delta?

You can answer this with dimensional analysis, which is just a fancy way of saying "make sure your units make sense".

So, let's say you have a speed. That's measured in metres per second (\$\frac m s\$). If we multiply by a time delta in seconds (\$s\$), we get \$\frac m s \times s = m\$ a value in metres (a displacement). That answers the question "How far should an object move, if it travelled at speed \$v\$ for duration \$\Delta t\$?"

But you're passing this to AddForce, which expects a force measured in Newtons (\$N = \frac {kg \cdot m} {s^2}\$), so these units do not make sense, and we know we wrote the wrong code here.

To convert a speed into a force, we need to know "how heavy is the object we want to accelerate up to that speed?" and "how long should it take to accelerate to that speed?"

So you could use code like this:

void FixedUpdate() { float mass = _rb.mass; float accelTime = 1f; // Replace with your preferred duration. Vector2 velocity = new Vector2(_speed, 0); Vector2 force = mass * velocity / (accelTime * accelTime); _rb.AddForce(force); } 

Running our dimensional analysis, you can confirm we end up with \$\frac {kg \cdot m} {s^2} = N\$, so our units make sense as a force now.

Notice that we did not need to use a time delta for the duration of the frame/physics step at all. That's because we're applying a force, which pushes just as hard at any simulation rate. The physics engine is then responsible for integrating that force over whatever duration it's simulating this tick to produce the appropriate velocity and position changes.

We mainly need to use time deltas when we're doing that integration ourselves, calculating our own change in velocity, position, charge, money, HP, or other quantity from a rate of change without delegating that calculation to the physics engine.