Question

Reading shown in two spring balances $S_1\text{and}{S}_2$ is $90\text{kg}$ and $30\text{kg}$ respectvely and lift is accelerating upwards with acceleration $10\frac{m}{s^2}.\text{If}{S}_1$ is elongated and $S_2$ is compressed and mass is stationary with respect to lift, then the mass of the block will be

• A. $60\text{kg}$
• B. $30\text{kg}$
• C. $120\text{kg}$
• D. None of these

Answer 1

The correct option is A $60\text{kg}$


As lift is accelerating upwards from $FBD$ of mass
$F_1+F_2-mg=ma$
$\therefore m=\frac{F_1+F_2}{(g+a)}$
Here $F_1$ is force acting on spring balance $S_1$. So, $F_1=90g$ and $F_2$ is force acting on spring balance $S_2$. So, $F_2=30g$.
Given that $a=10\frac{m}{s^2}$

$\Rightarrow m=\frac{(90+30)g}{(g+a)}=\frac{120\times 10}{10+10}=60\text{kg}$