NO net force is required to keep a moving object moving in a straight
line at a constant speed. In fact, if you apply ANY force to it, in ANY
direction, then its speed, its direction, or both must change, and its
velocity won't be uniform any more.
I know we never see this in our daily life. Whenever we see an object
moving, it always stops. That's because the net force on it is never zero ...
there's always some gravity or some friction acting on it. That's what you
have to put up with when you live on Earth.
