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.
It's a crucial question, because light and heat from the sun are the basis of (almost) all life on earth. Sunlight drives plant life via photosynthesis, and animals survive by eating plants. Almost all microscopic forms of life (bacteria, protozoa, etc.) survive by using the energy of sunlight.