. (i) How much
time does the ball take to return to his hands ? (ii) If the lift starts moving
up with a uniform speed of 
,
and the boy again throws the ball up with the maximum speed he can, how long
does the ball take to return to his hands ?
Answer:
(i) When the lift is stationary: For upward motion of the
ball, we have
As 
or 
.
As time of ascent = time of
descent
Total time
taken = 5 + 5 = 10 s.
(ii) When the lift moves up with
uniform speed : The uniform speed of the lift does not change the relative velocity
of the ball w.r.t. the boy i.e. it still remains 
.
Hence total time after which the ball
returns = 10 s.
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