Question

A uniform string of length `1.5` m has two successive
harmonics of frequencies 70 Hz and 84 Hz. The
speed of the wave in the string (in `ms^(-1)`) is
A. 84
B. 42
C. 21
D. `10.5`

Answer 1

Correct Answer - b
`because v_(n)=n/(2l)"v"` ...(i)
and `v_(n+1)=(n+1)/(2l)"v"` ...(ii)
From Equ. (i) and (ii) we get
`v_(n+1)-v_(n)=v/(2l)`
`rArr 84-70=v/(2l`
`therefore v=2 xx1.5xx14=42 " ms"^(-1)`