Question

A spring balance is attached to the ceiling of a lift. A man hangs his bag on the spring and the spring reads $49\text{N}$ when the lift is stationary. If the lift moves downward with an acceleration of $5{\text{m/s}}^2$, the reading of the spring balance will be (Take $g=9.8m/s^2$)

• A. $49\text{N}$
• B. $24\text{N}$
• C. $74\text{N}$
• D. $15\text{N}$

Answer 1

The correct option is B $24\text{N}$

When lift is stationary, tension in the string, $T=mg=49N\Rightarrow m=\frac{49}{g}$

Using Newton's second law,
$m\left(5\right)=mg-T_1\Rightarrow T_1=49-5\text{m}\Rightarrow T_1=49-5\left(\frac{49}{9.8}\right)$
$\Rightarrow T_1=49-5\times 5\Rightarrow T_1=24\text{N}$