## Physics (10th Edition)

Since the object is moving at a constant velocity, it has no acceleration, which means $\sum F=0$ $\sum F=0$ when either no forces are acting on the object or all the forces acting on the object have balanced one another. In b), when a single force $F$ acts on the object, it is not balanced by any other forces, so $\sum F=F\ne0$, so b) cannot be true.