A body of mass m is hanging from the hook of a spring balance and the balance is also hanging from the ceiling of a freely falling elevator. What will be the reading on balance?

The reading on the spring balance will be zero. Since the elevator is falling freely, the acceleration of the body $(a)$ becomes equal to the acceleration due to gravity $(g)$. So, the apparent weight of the body inside the elevator is, $W’=mg-ma=m(g-a)=m(g-g)=0$

Hence, the spring balance shows zero reading.