Question

An air-tight cage with a parrot sitting in it is suspended from the spring balance. The parrot starts flying. The reading of the spring balance will
A. increase
B. decrease
C. not change
D. be zero

Answer 1

A spring balance is a type of mechanical device that is used for measuring weight of an object by using the tension force created on the spring. It consists of a spring whose one end is fixed to a rigid support and the other end has a hook attached to it on which the object whose weight is to be measured is suspended.
When the bird starts flying in the air tight container, it pushes some air in downward direction with a force equal to its own weight.

Complete step by step answer:
Let us first discuss spring balance.
A spring balance is a type of mechanical device that is used for measuring weight of an object by using the tension force created on the spring. It consists of a spring whose one end is fixed to a rigid support and the other end has a hook attached to it on which the object whose weight is to be measured is suspended.
As given in the question, the parrot is sitting in an airtight cage on a spring balance whose one end is fixed to the cage. In this case, the spring balance reading will be the weight of the parrot and the cage.
Now, when the bird starts flying in the air tight container, it pushes some air in downward direction with a force equal to its own weight. This will increase the weight of the base of the cage by the amount equal to the weight of the parrot. In this case, the spring balance will equal the weight of the whole cage which became the same as in the previous case.
Therefore, the reading of the spring balance will not change and remains the same.
Hence, option C is correct.

Note: A spring balance reading will be correct (actual) only if it is measured with respect to a frame of reference where the acceleration in the spring axis is constant (such as on earth, where the acceleration is due to gravity). This can be verified by taking a spring balance into a lift, where the weight measured will change as the lift moves up and down with changing velocities.