Final answer:
Fish and humans are propelled forward by external forces exerted by the medium they are in. Fish push water backward with their fins, while humans can be propelled forward by pulling on a rope.
Step-by-step explanation:
The fish pushes water backward with its fins, which is how it propels itself forward. The water exerts an external force on the fish in the opposite direction, pushing the fish forward (b).
Similarly, humans can propel themselves forward by using external forces. For example, when a person pulls down on a vertical rope, the rope pulls up on the person, propelling the person forward (Figure 5.17).