We are trying to calculate the acceleration of gravity using a quarter inch ball bearing and a high speed video camera. The ball bearing it shot at 6 m/s across a grid .5 meters long. The high speed camera is 1.8 meters away and records the position of the ball at 250 frames per second. We thought that we could determine how fast the ball was accelerating downwards by analyzing the position of the ball bearing on each frame. However, we are systematically getting lower than usual results (9 m/s/s instead of 9.8 m/s/s). We have made sure that the set up is as precise as possible using laser levels. We have also checked the timing mechanism of the camera's clock. Does anyone have any suggestions?