Solution:
SOLUTION
First ball is in air for time t0 + t and for it
u = 0, a = g

Second ball is in air for time t and for it
u = 0, a = g

∴ Distance between balls at time t

The equation for s is linear in t (y = mx + c), so graph of s versus t is a straight line.
After ball 1 reaches the ground it stays there
Hence, distance between balls =1/2gt2
s =g/2 t2 (parabola opening upwards).