What is the net force on a bag if it's pulled down by gravity with a force of 18 newtons and pulled up by a rope with a force of 18 newtons?

The correct choice is zero newtons because the forces acting on the bag are balanced. When the bag is pulled down by gravity with a force of 18 newtons and simultaneously pulled up by a rope with a force of 18 newtons, these two forces are equal in magnitude but opposite in direction. According to Newton's first law of motion, if the net force acting on an object is zero, the object will remain at rest or move at a constant velocity. In this case, since the upward and downward forces cancel each other out perfectly, the net force on the bag is indeed zero newtons. This indicates that the bag will not accelerate in either direction, remaining in a state of equilibrium.