As weird as it sounds, NO net force is required to keep a moving object
moving in a straight line at a constant speed. In fact, if ANY net force
suddenly shows up, then the object CAN't keep moving at the same speed
in the same direction. Something would have to change.
Force is only required to CHANGE the object's motion ... make it speed up,
slow down, or turn to a different direction. Those are the changes that are
called "acceleration", and THAT's where force is needed.
The answer to the question is: Zero .
