If we model the motion of a bouncing ball on a velocity time graph, neglecting air resistance, we get a line with gradient -9.8 as the ball falls, and then a steep positive gradient line when the ball hits the floor and then the same -9.8 gradient line again as the ball goes up. What I don't understand is if we take the downwards direction as positive, then shouldn't the ball have a gradient of 9.8 as it falls and then a gradient of -9.8 as it goes up? The same question for acceleration time graphs as well. Moreover, Why is there a sudden steep gradient at the impact with the floor. I hope someone can talk me through the velocity-time and acceleration-time graphs for a bouncing ball neglecting air resistance. My calculus skills are limited, but I do understand the concepts of differentiation and integration and how velocity is the derivative of displacement and so on.