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Question

A pebble is thrown vertically upwards from bridge with an initial velocity of 4.9 m/s. It strikes the water after 2s. If acceleration due to gravity is 9.8 m/s2. The height of the bridge & velocity with which the pebble strikes the water will respectively be-

A
4.9 m, 1.47 m/s
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B
9.8 m, 14.7 m/s
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C
49 m, 1.47 m/s
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D
1.47 m, 4.9 m/s
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Solution

The correct option is B 9.8 m, 14.7 m/s
Given data
u=4.9m/s
a=9.8m/s
t=2s
Taking the point of projection as origin and downward direction as positive,
By second equation of motion, i.e.,
s=ut+12at2
s=h,aftert=2s
h=4.9×2+12(9.8)(2)2
h=9.82(9.8)
h=9.8m
|h|=9.8m
velocity after 2s
By first equation of motion, i.e.,
v=u+at
v=4.9+(9.8)(2)
v=14.7m/s


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