Two stones are thrown up simultaneously from the edge of cliff with...

Initially when both are in air, the relative acceleration between them is zero. Therefore, relative position verus-time graph is a straight line.

${S}_{1} = vt - \dfrac{1}{2}gt^{2}$

${S}_{2} = 2vt - \dfrac{1}{2}gt^{2}$

${S}_{21} = {S}_{2} - {S}_{1}$

${S}_{21} = vt$

Once the first ball reaches the ground, its acceleration becomes zero. Therefore, the relative acceleration is now g and the graph is parabolic.

${S}_{2} = 2vt - \dfrac{1}{2}gt^{2}$