Which formulas might be used to find the velocity based on a position-time graph?

The formula to get the velocity is V=D/T. First you need to determine the distance and time at a certain point and plug it into the equation and that will be your velocity at that point. To get it between two points you complete the directions two times and find the average or use the slope formula, slope = rise/run; slope = (y2 - y1) / (x2 - x1). 

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.