Question

In the figure the string has a mass $4.5g$. How much time will it take for a transverse disturbance produced at the floor to reach the pulley?
(Take $g=10m{s}^{-2}$)

• A. $0.04s$
• B. $0.2s$
• C. $0.4s$
• D. $0.02s$

Answer 1

The correct option is D $0.02s$

$v=\sqrt{\frac{T}{\mu }}=\sqrt{\frac{20}{2\times {10}^{-3}}}=100m{s}^{-1}$
$\mu =\frac{4.5}{2.25}=2g{m}^{-1}$
$s=ut-\frac{1}{2}g{t}^2\Rightarrow 2=100t-5t^2$
$\Rightarrow t^2-20t+0.4=0$
$t=\frac{+20\pm \sqrt{400-1.6}}{2}=0.02s$