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.
If this seems a useless cycle, remember that a plant does this to provide raw materials for its own use, including generating energy when light is not available, and animals consume the materials created by plants to provide for their needs. In effect, animals are living indirectly on the energy of the sun's radiation Photosynthesis in plants