Question

A man generates a symmetrical pulse i a string by moving his hand up and down. At $t=0$ the point in his hand moves downward. The pulses travels with speed of $3m/s$ on the string & his hands passes $6$ times in each second from the mean position. Then the point on the string at a distance $3m$ will reached its upper extreme first time at time $t=$

• A. $1.25sec.$
• B. $1sec.$
• C. $\frac{13}{12}\sec$
• D. None

Answer 1

The correct option is B $1sec.$

If hand power 6 times

from the mean position in

one second then we know that

3 wave length or 3 cycles for 1 second

$\lambda =\frac{v}{f}=\frac{3}{3}=1m$

If $\lambda =1m$ the point which have

3 m distance located at no. 6 in the fig

to reach extreme if have to

travel $\frac{3\lambda }{u}$ distance

Time to travel $3\lambda =1sec$

$\lambda =1/3$

Time to Travel $\frac{3\lambda }{4}=\frac{3\left(1/3\right)}{4}=\frac{1}{u}=0.25sec$