This is an example of how to model the simple trajectory of a potato discharged from a potato cannon. To do this you need to have the gravitational constant, velocity of the projectile as it leaves the barrel, and the angle between the barrel and the ground.
Step 1: Finding Velocity
My first step is to find the velocity. I used the method of finding the distanced traveled per frames. Knowing the frame rate of the camera, the velocity can be calculated. In my case I calculated, as seen in the video, the velocity to be 60ft/s since it traveled 5ft in 5 frames while being filmed at 60 frames/s.
5ft * (1/5 frames) * 60 frames/s = 60ft/s