Question

A stone A is thrown vertically upward with speed U and another stone B is thrown vertically downward with same speed you 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

Answer 1

Given, time taken by stone A is 16s

Initial speed = u

Time taken by stone B is 4s

Initial speed = u

final velocity = 0

V = u+at

0 = u+ 10*40

Therefore u for second stone is -40m/s

As the height is same for both the stones,

S = u*16+1/2*-10*16*16 for stone 1 ----(1)

S = -40*4+1/2*10*4*4 ----- (2)

Now, equating eqn. 1 and 2

u*16+1/2*-10*16*16 = -40*4+1/2*10*4*4

u = 75m/s for stone 2

Now, s = 75*16+1/2*-10*16*16

S = -80m

Now for stone 3,

v = 0,

v^2 - u^2 = 2as

0 - u^2 = 2*10*-80

Therefore u = 400m/s

Now, v = u+at

0 = 400+10*t

Therefore time taken by third stone is 40s.