1. How to get x,y positions for a tennis ball at each time t given total force exerted? 2. We consider Drag force D, Lift Force L and Gravitation force W D=1/2*(0.55+1/((22.5+4.2*(v/(w*R))^2.5)^0.4))*pi*R^2*d*v^2 L=1/2*(1/(2+(v/(w*R))))*pi*R^2*d*v^2 W=m*g as described in http://www.physics.usyd.edu.au/~cross/TRAJECTORIES/42. Ball Trajectories.pdf where v, speed; w, angular speed;R, radius;d, density;m, mass 3. I suppose we need to integrate force twice on time t for Fx and Fy given a Serve Angle Ro and Serve Speed vi, but how?