In the latter case the resistance is caused by dissipation of mechanical energy when the configuration or the direction of flow is sharply changed, by the formation of vortices and secondary flows as a result of the flow breaking away, by the centrifugal forces, etc.
Animals depend on plants for food as plants produce food and energy which animals can't, so, herbivores eat plants to get energy, carnivores eat herbivores and other carnivores for energy and omnivores eat all four, plants, herbivores, carnivores and also other omnivores for energy.
Because animals use organic compounds as food that are produces by plants. Animals depend on plants to survive because the plants give them a  bunch of resources. They might eat them, or make their nests, or  beds out of them. There are animals that eat only plants for food.