First slide

Constraints

Question

Two masses of I 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:

Moderate

A

more than 6 kg
B

less than 6 kg
C

equal to 6 kg
D

none of the above

Solution

Suppose T be the tension in each part of the string passing over the pulley. Iis given by:
$T = \frac{2 m_{1} m_{2} g}{m_{1} + m_{2}} = \frac{2 \times 1 \times 5}{1 + 5} g = \frac{5}{3} g$
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 = $\frac{10}{3} K g f$ . So reading will be $\frac{10}{3}$ kgf which is less than 6 kgf .

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