how to make something happen if velocity reaches a certain value?

I need to know how to make something happen after a rigidbody reaches a certain velocity,

Adding onto what Kudorado said, say you have a Rigidbody variable (reference to a Rigidbody component) stored in Rigidbody r. Then you can use r.velocity, which will return a Vector3 back to you.

You can check the velocity (with signage to show direction – positive and negative numbers are both allowed) in each axis individually with r.velocity.x, r.velocity.y, and r.velocity.z.

But if you want to know how fast something is moving (speed, which is always positive), then you want to find the length of the velocity vector (the length of a vector is its size – how big it is in overall). You can get the length of any Vector3 by calling .magnitude on the vector – so that’d be r.velocity.magnitude.

And of course it’s very open ended – you can do anything you’d like with that value. So you can throw it in an if statement if you want in the Update method as an example:

public Rigidbody r;

public void Update() {
	float speed = r.velocity.magnitude;
	if (speed >= 3) {
		//Do something
	}
}

I hope this helps!

Depending on which axis do you want to check, velocity will return a Vector3 on Rigidbody, Vector2 on Rigidbody2D.
you can access velocity via Rigidbody.velocity.