A bird is sitting in a large closed cage which is placed on a spring balance. It records a weight placed on a spring balance. It records a weight of 25 N. The bird (mass = 0.5kg) flies upward in the cage with an acceleration of 2m /s² . The spring balance will now record a weight of

The correct option is 25 N.

Let us analyze the system to understand why the weight recorded by the spring balance remains unchanged when the bird flies inside the cage.

1. Understanding the System:
The bird is inside a closed cage. The key term here is "closed cage".
When a bird is inside a completely closed container, the air inside is trapped and moves along with the container as part of a single, closed system.

2. Mechanism of Flight:
For the bird to fly upward with an acceleration, it must push the air downwards with its wings. According to Newton's third law of motion (action and reaction), the air exerts an equal and opposite upward force on the bird, allowing it to accelerate upward.
Specifically, to accelerate upward with acceleration $a$, the wings of the bird of mass $m$ must push down on the air with an additional force equal to $ma$.

3. Force Transmission in a Closed Container:
This downward force exerted by the bird's wings is transmitted directly to the air column inside the cage.
Since the cage is closed, this downward force on the air is eventually transmitted to the bottom floor of the cage.
Therefore, the downward force on the cage floor increases by exactly the force required to accelerate the bird upward:
$F_{\text{downward}}=m(g+a)$
where $g$ is the acceleration due to gravity.

4. Net Force on the Spring Balance:
The spring balance measures the total force exerted on it by the cage-bird-air system.
For the entire closed system, all the forces involved in the bird's acceleration (the upward lift force on the bird and the downward reaction force on the air and cage floor) are internal forces.
Since internal forces cannot change the total weight recorded by the scale of a closed system, the net downward force acting on the spring balance remains exactly equal to the total static weight of the cage and the bird.

Thus, the spring balance will continue to record a weight of 25 N.