Q.

A particle initially (i.e, at t = 0) moving with a velocity u is subjected to a retarding force, as a result of which it decelerates at a rate a = - kv where v is the instantaneous velocity and k is a positive constant. The time T taken by the particle to come to rest is given by

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a

T = 2u32k

b

T = 2uk

c

T = 2ukv

d

T = 2u2k

answer is A.

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Detailed Solution

Given a = - kv or  dvdt = kv . Thus v-12 dv = -k dt  Integrating, we get 2v12 =-kt+c. Using the given initial condition  (v = u at  t = 0), we get c = 2u . Thus, we have

2v12 -u12-kt

Now, use t = T and v = 0.

we get,

T=2uk

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