Add to your main loop handling for MOUSEBUTTONUP and MOUSEBUTTONDOWN events. When MOUSEBUTTONDOWN or MOUSEBUTTONUP occurs: Store the respective mouse location On MOUSEBUTTONUP Compute the initial x-velocity, xv, as the difference in the x-coordinates when the mouse was depressed vs released. *Compute the initial y-velocity, yv, as negative one times the difference in the y-coordinates when the mouse was depressed vs released. Remember: The positive y-axis points down in Pygame.