S1=10t−21gt2. When S1=−240⇒t=8s. So, at t=8 seconds first stone will reach the ground. S2=40t−21gt2. Till t=8s, S2−S1=30t. But after 8s S1 is constant =−240. Relative to stone 1 for (t1>8)s displacements of stone 2 , S2+240. And at t=12s seconds stone will reach the ground. The corresponding graph of the relative position of second stone with respect to first is,




