This is my first post to this forum! I'm working on a Lab write up for introductory physics involving projectile motion and velocity. For the experiment we threw a ball such that its motion was as close to parabolic as possible. A video was taken and using a video analysis program we were tasked with determining whether or not the horizontal component of velocity remains constant - we found this to be true, as well as the equation which described the vertical position of the ball. The vertical position of the ball was found to be of the form: y = yo + voyt + (0.5)ayt2 The problem is ay, which should be -9.8 m/s2 was found to be -11.4 m/s2. The data we collected is a perfect fit with our match equation but I cannot for the life of me figure out why the value for ay would be off by so much. The only thing that I can think of is that maybe the spin of the ball while it's in motion is somehow causing the discrepancy between values. Is this a possibility? Any other explanations would be great as well! Thanks!