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# CBSE Class 9 Answered

A ball is gently dropped from a height of 20 m. If its velocity increases uniformly at the rate of 10 m s-2, with what velocity willA ball is gently dropped from a height of 20 m. If its velocity increases uniformly at the rate of 10 m s-2, with what velocity will it strike the ground? After what time will it strike the ground? it strike the ground? After what time will it strike the ground?

Asked by jeevika.9a | 13 Feb, 2021, 06:00: PM
It is given that velocity increases at the rate of 10 m/s2 , i.e., acceleration is 10 m/s2 which is very near to the standard vlue
of acceleration due to gravity g = 9.8 m/s2 . In physics problems, for the convenience of doing calculation ,
it is the standard practise to consider acceleration due to gravity g = 10 m/s2 .

Hence , if rate of increase of velocity is 10 m/s2 , then when the ball is dropped, it has started from rest , i.e., initial velocity is zero.

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Velocity v of ball when it reaches ground is determined from the following equation

" v2 = u2 + ( 2 g h ) "

where u = 0 is initial speed , g = 10 m/s2 is acceleration due to gravity and h is height from where the ball is dropped

v2 = 2 × 10 × 20 = 400 m2 /s2    or    v = 20 m/s

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Time taken t by the ball to reach the ground is determined from the following equation

" h = ( u t ) + [ (1/2) g t2 ]  "

20 = 0.5 × 10 × t2   or   t = 2 s

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Answered by Thiyagarajan K | 13 Feb, 2021, 07:43: PM

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