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A string of length 0.4 m and mass 102 kg is tightly clampled at its ends. The tension in the string is 1.6 N. identical wave pulses are produced at one end at equal intervals of time ∆t. The minimum value of ∆t, which allows constructive interference between succesive pulses, is :
Options:
A .  0.05 s
B .  0.10 s
C .  0.20 s
D .  0.40 s
Answer: Option B
:
B
Mass per unit length of the string,
m=1020.4=2.5×102 kg/m
Velocity of wave in the string.
v=Tm=1.62.5×102
v=8m/s
For constructive interference between successive pulses:
tmin=2lv=(2)(0.4)8=0.10s
(After two reflections, the wave pulse is in same phase as it was produced, since in one reflection its phase changes by π and if at this moment next identical pulse is produced, then constructive interference will be obtained.)

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