The two velocities add on top of each other.
You can imagine the normal velocity as the base-velocity. The particles will ALWAYS be emitted at -1 m/s relative to the normal of our emitter.
The object velocity is a velocity that’s added on top of our base-velocity. As the rocket (and inherently, the emitter) gains velocity, the particles will be emitted at -1 * n m/s, where n is our objects velocity.
So our sum velocity is:
velocity = normal_velocity + object_velocity
(-(1 + n ) m/s) = (-1 m/s) + (-1 * n m/s), where n is the velocity of our rocket
