Rigidbody2D and AddForceAtPosition

Im trying to make a rocket, and im using an empty gameobject parented to the rocket for the motor position, the point where the force will be applied, here is my code: motorPivot = GameObject.FindGameObjectWithTag("motorPivot").transform.position; rigidbody2D.AddForceAtPosition(transform.up * force,motorPivot); Even with the motorPivot exactly equal to the transform.position of the rocket, im getting diferent results when i put transform.position instead of the motorPivot. With transform.position, the rocket move upwards regardless of the world pos of the rocket (that is correct). With the motorPivot (wich is exactly the same values of the transform.position), the rocket slight rotate twards the center of the world.

Whats wrong?