A string of length 1.5 m with its two ends clamped is vibrating in fundamental mode. Amplitude at the centre of the string is 4 mm. Minimum distance between the two points having amplitude 2 mm is:

Explanation

`lambda=2l=3m`
Equation of standing wave
`y=2A sin kx cos omegat`
`y=A` as amplitude is 2A.
`A=2A sin kx`
`(2pi)/lambda x=pi/6 rArr x_1=1/4 m`
and `(2pi)/lambda. X=(5pi)/6 rArr x_2=1.25 m rArr x_2-x_1=1 m`