For the upward motion, the velocity decreases uniformly with retardation g , where g is the acceleration due to gravity. The line AB in Fig represents this motion. <br> <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/CHY_DMB_PHY_XI_P1_U02_C01_E08_006_S01.png" width="80%"> <br> Then for the downward motion the velocity increases g. The line BC in the same figure represents this motion. <br> (ii) The entire motion is under the acceleration due to gravity (g) which is a constant . So the acceleration-time graph is a horizontal straight line Fig.