1.

The amplitude of a damped oscillator decreases to 0.9 times its original magnitude in 5s. In another 10 s it will decrease to α times its original magnitude, then α is equal to :(a) 0.7 (b) 0.81 (c) 0.729 (d) 0.6

Answer»

Answer : (c) 0.729 

The amplitude of damped oscillator is,

A = A0e-kt

Where k = \(\frac{b}{2m}\) 

At t = 5s, 

0.9 A0 = A0e-5k

⇒ e-5k = 0.9

At t = 15s, 

αA0 = A0e-15k

⇒ e-15k = α

⇒ e(-5k)3 = α

⇒ α = (0.9)3

= 0.729



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