Question

A stone A is thrown vertically upward with speed U and another stone B is thrown vertically downward with same speed u from a height h the time taken by stone A and B to reach the ground at 16 second and 4 seconds respectively the time taken by another stone to reach the ground after it has been dropped from height H is. Ans 8 s

Answer 1

Let say initial velocity = U m/s

Stone B reaches ground in 4 seconds

S = ut + (1/2)at²

a = g (acceleration due to gravity)

H = 4U  + (1/2)g(4)²

=> H = 4U + 8g

Stone A is thrown upward

it will take 8 sec to go up & 8 sec to come down

0 = U - g8  ( going upward)

=> U = 8g

H = 4*8g + 8g

=> H = 40 g

Stone is dropped from height 40g

u = 0

H = (1/2)gt²

=> 40g = (1/2)gt²

=> t² = 80

=> t = 8.94 sec