Question

In the figure a pulley of negligible weight is suspended by a spring balance S Masses of 3 kg and 7 kg respectively are attached to opposite ends of a string passing over a pulley P The spring balance reads:

• A. equal to 10 kg
• B. less than 10 kg
• C. more than 10 kg
• D. equal to 4 kg

Answer 1

The correct option is B less than 10 kg

As two object are connected to the pulley

We can calculated by it by using FBD

$\begin{array}{c}accordingtoFBD\\ T-30=F.....\left(eq1\right)\\ 70-T=F......\left(eq2\right)\end{array}$

as spring balance reading the equilibrium position , the net force will be $0$.Now equating both equation

we get,

$\begin{array}{c}T-30=70-7\\ T+T=100\\ 2T=100\\ T=50N\end{array}$

now we have to calculate the reading

we know

$\begin{array}{c}T=F\\ F=mg\\ 50=m\left(10\right)\\ m=5kg\end{array}$

The reading of the spring balance will be $5kg$

Hence,

option $B$ is correct answer.