A body dropped from a height h with an initial speed zero, strikes the ground with a velocity `3km/h`. Another body of same mass is dropped from the same height h with an initial speed `-u' = 4km//h`. Find the final velocity of second body with which it strikes the ground

Options

A. 3km/hr
B. 4km/hr
C. 5km/hr
D. 6 km/hr

Answer

C

Explanation

From third equation of motion, `v^(2) + u^(2) + 2as` where v & u are final & initial velocity, a is acceleration, s is distance.
for first case `v_(1) =` 3km/hour, `u_(1) = 0, a_(1) =g "& " s_(1)=?`
`s_(1) = (9 xx 100)/(36 xx 36 xx 20)` metre
For second case `v_(2) =?, u_(2) =`4km/hour,
`a_(2) = g = 10 m//sec` & `s_(1) = s_(2) = (9 xx 100)/(36 xx 36 xx 20)`
so `v_(2)^(2) = (16 xx 1000 xx 1000)/(3600 xx 3600) + (2 xx 10 xx 9 xx 100)/(20 xx 36 xx 36)`
or `v_(2) =5` km/hour