Question

A man generates a symmetrical pulse in a string by moving his hand up and down . At t = 0 the point in his hand moves downward. the pulse travels with speed of 3 m/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 reach its upper extreme first time at times t =

• A. $1.25sec$
• B. $1sec$
• C. $\frac{13}{12}\sec$
• D. $0.25$secs

Answer 1

The correct option is D $0.25$secs

The man's hand passes $6$ times from the mean position in $1$ sec, we can find that string creates $3$ cycles after $1$ sec.

Frequency of wave= $3Hz$

$V=f\lambda \Rightarrow \lambda =\frac{V}{f}=\frac{3}{3}=1m$

To reach upper extreme $⟶$ have to travel $3\lambda /4$ distance.

Time to travel $\frac{3\lambda }{4}=\frac{3}{1}\times \frac{1}{3}=\frac{1}{4}=0.25$ sec