Why does using a temporary variable break rigidbody?

Questions & Answers
1

So, this is kind of an odd situation; I have two scripts attached to a rigidbody. One script will apply a horizontal force when the player presses a button:

// Apply a force in a direction.
rigidbody.AddForce(dir * power, ForceMode.Impulse);

And another has logic for limiting the velocity of the rigidbody:

Vector3 tempVelocity = rigidbody.velocity;
if(maxVelocitySq > 0.0f && tempVelocity.sqrMagnitude > maxVelocitySq)
{
    tempVelocity *= slowdownRate;
}
rigidbody.velocity = tempVelocity;

However, when I run the game, the horizontal force doesn’t work. I thought that there might be some sort of conflict or overwriting of the rigidbody’s velocity force based on the update order of these two scripts, but then I found that I could fix the problem by NOT using a temporary variable in the second script:

// If velocity is too high, apply a slowdown.
if(maxVelocitySq > 0.0f && rigidbody.velocity.sqrMagnitude > maxVelocitySq)
{	
    rigidbody.velocity *= slowdownRate;
}

And that second version works fine.

My question, then: why would using a temporary variable break the AddForce line in the first script? Does Unity execute physics updates concurrently on different threads, so using a temp variable might overwrite some data? Is there something wrong with using a temp variable? Or is it a bug that rigidbodies would behave this way? I’d note that I’m working on an iOS/Android title, but I only ever see this behavior in the editor.

2

So, I never really found an answer to why a temporary variable would cause such unexpected physics issues. However, I suppose a valid solution to temporary variables breaking physics is to not use a temporary variable:

// Limit the player's fall velocity.
if(rigidbody.velocity.z < maxFallVelocity)
{
	float diff = rigidbody.velocity.z - maxFallVelocity;
	Vector3 force = new Vector3(0.0f, 0.0f, -diff);
					
	rigidbody.AddForce(force, ForceMode.VelocityChange);
}

This seems to do the trick by calculating an opposing force on the player and applying it using good old “AddForce”. Note that the game I’m working on has gravity on the Z-axis (due to nav-mesh limitations), so you’d probably want to change those Zs to Ys if you copy this code.