unity cant change rigidbody speed of another object code example

Example: get an object speed unity rigidbody

float maxSpeed = 1.0f; // units/sec void FixedUpdate() {     Rigidbody rb = GetComponent<Rigidbody>();     Vector3 vel = rb.velocity;     if (vel.magnitude > maxSpeed) {         rb.velocity = vel.normalized * maxSpeed;     } }