Question

why do we feel hungry after a physical activity

Answer 1

When we perform physical activity, tremendous amount of energy is utilised by the body, this energy comes from the food stored in our body. since, a large amount of food reserves get burnt up during exercise, we feel hungry after it.
During physical activity we require more energy. To fulfill this energy requirement a person breathes faster. As a result more oxygen is supplied to our cells. Due to this rate of breakdown of food increases and more energy is released. Due to rapid breakdown of food we feel hungry.

Answer 2

while doing physical activity we use much energy and we need more energy. so we feel hungry .