Two basketballs were thrown towards each other and collided in mid air. A video was made of the process and used as input for the "World in Motion" program. The paths of the basketballs were marked on the video and after inputting length and time scales and the mass of the basketball the program computed graphs showing the time variation of momentum and energy. The observed graphs were compared to ones simulating the motion produced in the "Interactive Physics" program. This allowed us to determine what forces were acting on the basketballs.
The above image shows the data points made with each step and some data relating to the objects in the image. The meter rule on the ground benchmarks distance for the "World in Motion" program. Time is benchmarked by setting the Frames Per Second (FPS) in "World in Motion" to the FPS rate at which the video was made.
This graph shows the momentum in the X direction (red and green) of both balls ( 1 and 2) and the total momentum in the X direction (blue). The point where the lines cross (approximately zero momentum for each ball) is the point where the balls collide with each other. It can be seen from this graph that total momentum in the X direction varies slowly during the experiment. The individual momenta are decreasing in magnitude with time before the collision and stop increasing or decrease after the momentum changes produced during the collision.
This video from Interactive Physics shows a simulation of the action we performed. The graphs in this video show that neither momentum in the X nor Y directions are conserved just as it was in the World in Motion video. It also shows that the Y momentum is distinctly altered by outside forces just as in the experiment from above.
The graphs show the change of energy of the balls during the motion, clearly the total energy is not conserved.
The Interactive Physics program has the gravity force set in a vertical direction (downwards) with a magnitude that corresponds to normal earth gravity, and air resistance set as normal atmospheric air resistance (for earth).
Without air resistance one sees that the velocity in the x-direction is constant before and after the collision, this is not observed in our experiment thus one can conclude that air resistance is present.
Without gravity but with air resistance the velocity in the x and y directions just slowly decrease before and after the collision. This was not true for the velocity in the y-direction in the experiment, which decreased quickly all the time, showing the presence of a downward gravity force.
Here we see the idealized situation of no external forces during the collision process, the velocity in both the x and y direction stays constant after the collision showing the absence of all external forces.