Question

A bird on the floor of an airtight box which is being carried by a boy starts flying. The boy feels that the box is now:
A) Lighter
B) Heavier
C) Shows no change in weight
D) Lighter in beginning and then heavier

Answer 1

Hint: Since the box is air tight no air can pass through it. So if a bird is sitting then the total weight is equal to the sum of weight of box and bird. And if the bird is flying, the reaction force applied on the floor of the box will be equal to its weight. So there will be no change in total weight of the box. The reaction of the bird does not change as the box is air tight.

Complete step by step answer:
If a bird starts flying in an airtight box then the total weight of the box does not change. Whenever birds fly they have to apply force downwards with the help of their wings. The force creates an opposite force which lifts a bird upwards. This upward force is called the reaction force. When there is no place for air to escape, then the reaction force is the same as the weight of the bird. The reaction force is applied everywhere in the surroundings. For ex- humans and animals can move and run due to the reaction force applied by the earth. Without a reaction force nobody can move or stop.

In zero gravity, no body can move until and unless any external force is applied. On the basis of this principle, Newton gave the 3rd law of motion which stated that- every action has an equal and opposite reaction. This means that whenever a force is applied on another body an equal and opposite reaction force is given by the body. Here to make a bird fly a force equal to its weight as to be applied. And reaction force equal to its weight is generated. So no change in weight is felt by the boy.

Note: The bird is placed in an airtight box. So when a force is applied by it to fly an equal force is given by the floor of the box. Due to this the total weight of the box does not change. This reaction is equal to the weight of the bird. The boy does not feel any change in the weight of the box.