A bird is sitting in a wire cage hanging from the spring balance. Let the reading of the spring balance be a. If the bird flies about inside the cage the reading of the spring balance is b. Which of the following options is true:
A. \[a=b\]
B. \[a > b\]
C. \[a\]
D. Cannot be obtained

Hint:This question is based on Newton's 3rd law of motion, which states that every action has an equal and opposite reaction.

Complete step by step solution:
Reading of weighing machine\[=N\]
Free body Diagram:
 

 
\[N=\]Force on the weighing machine due to the bird which is given.
\[{{N}_{1}}=a\](Given)
But in second case,
If a bird is flying in the cage. It applied the air pressure on the weighing machine, so that the bird can fly again.
Here,
\[N=\]Force due to air pressure of the wings on the weighing machine.
So, \[{{N}_{2}}=b\]
Then, \[{{N}_{1}}={{N}_{2}}\]
Hence, \[a=b\]
So, the option (A) is correct option

Note:Study about the properties of the weighing machine and Newton’s 3rd law of motion, to get the proper idea of this question.