Question

Find the reading of the spring balance considering spring, pulley and string to be massless.

• A. $600\text{N}$
• B. $300\text{N}$
• C. $200\text{N}$
• D. $400\text{N}$

Answer 1

The correct option is B $300\text{N}$

The spring balance will read the tension in the string. Let the common acceleration is $a$.
$FBD$ of the two blocks and the spring balance,

On applying $\sum F=ma$ on the two blocks and the spring, we get,
$600-T_1=60a$ .............(1)
$T_1-T_2=0$ ............(2)
$T_2-200=20a$ .............(3)
On adding all the equations, we get,
$a=5{\text{m/s}}^2$, putting this in (1), we get,
$T_1=300\text{N}$
$T_1=T_2=300\text{N}$ [from (2)]