A ball falls from height h. After 1 s, another ball falls freely from a point 25m below the point from where the first ball falls. Both of them reach the ground at the same time. The value of h is

a) 11.2 m

b) 21.2 m

c) 31.2 m

d) 41.2 m

Answer: c) 31.2 m

Solution:

For the first ball, h = (1/2)gt² ——-(1)

For the second ball, (h – 20) = (1/2)g(t-1)² ——-(2)

From equation (1) and (2)

gt²/2 – 20 = (g/2)(t² -2t +1)

gt = (g/2) + 20

t = 25/10 = 2.5

h = (1/2)gt²

= (1/2) x 10 x (2.5)²

= 31.2 m