Question

only physics iit jee expert can solve this ..................,
a ball thrown vertically upwards with a speed u from the top of a tower reaches the ground in 9 seconds. Another ball thrown vertically downwards
from the same position with the same speed 'u', takes
4 seconds to reach ground. Calculate the value of 'u'.
(Take g = 10 m s-2)​

Answer 1

Answer:

25 m/s

Explanation:

let the upward direction be positive...

case 1:

u=+u m/s

s=-h m

t=9 s

s=ut+1/2at²

-h=u9+1/2*(-10)*9²

=>-h=9u-405 .......(equ (i))

case 2:

u=-u m/s

s=-h m

t=4 s

s=ut+1/2at²

-h=-4u+1/2*(-10)*4²

=>-h=-4u-80....….(equ (ii))

now subtracting equ)ii) from equ(i)

we get,

=>0=13 u -325

=>u=+25 m/s

[positive sign imply that the direction of intial velocity was upwards]