Question

Two masses of $1 \,kg$ and $5\, kg$ are attached to the ends of a massless string passing over a pulley of negligible weight. The pulley itself is attached to a light spring balance as shown in figure. The masses start moving during this interval; the reading of spring balance will be:

• A. more than $6 \,kg$
• B. less than $6\, kg$
• C. equal to $6\, kg$
• D. none of the above

Answer 1

Answer: (B)

Suppose The the tension in each part of the string passing over the pulley. T is given by:
$T=\frac{2m_{1}m_{2}g}{m_{1}+m_{2}}$
$=\frac{2 \times 1 \times 5}{1+5} g=\frac{5}{3}g$
For equilibrium of the pulley there is an upward force of $2T$ acting on the string joining the pulley to the spring. Therefore, reading of spring balance will be $2T=10g/3$ . So reading will be $10/3 \,kg$ which is less than $6\,kg$