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Question

A ball is thrown horizontally from a height of 20 m. It hits the ground with a velocity three times its initial velocity. The initial velocity of ball is

A
2 m/s
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B
3 m/s
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C
5 m/s
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D
7 m/s
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Solution

The correct option is D 7 m/s

Given that,

Initial speed = u

Height = 20 m


This speed will remain constant throughout the journey.

Horizontal component of velocity when it hits the ground is

vx=u


if vy be the vertical component of velocity when it hits the ground then,

v2y=2gh

v2y=2×9.8×20

v2y=392m2/s2


Now, final speed of object

=v2x+v2y

=u+392


Now, according to question

u+392=3u

9u2=u2+392

8u2=392

u2=49

u=7m/s

Hence, the final velocity is 7 m/s


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