A rocket moves through empty space in a straight line with constant speed. It is far from the gravitational effect of any star or planet. Under these conditions, the force that must be applied to the rocket in order to sustain its motion is

Newton's three laws:
Newton's first law: Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.
Newton's second law: The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object.
Newton's third law: Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.