1. Hi, I'm a computer engineering graduate. I'm developing a computer game similar to Soundrop (Apple). The situation is: balls fall vertically down under the effect of gravity. There are various inclined planes in the frame (of known angle of inclination). There could be multiple collisions possible and all collisions are considered to be perfectly elastic. I'm able to create a trajectory of flight for the balls till the next point of collision. However I'm unable to accurately determine the angle of reflection from the next incline.1 2. Relevant equations I've used the following equations in my calculations of the trajectory: x= x_initial + u_x_initial*T (T=time of flight till collision) y=y_initial + u_y_initial*T - 0.5*g*T^2 I've also considered that the angle of incidence of a ball on the incline with respect to the normal to the plane is equal to the angle of reflection. Thus for an incline with angle of inclination less than 90 (like this: ∠), I've considered the following: θ= angle of inclination γ= angle of incidence with respect to the horizontal α= angle of reflection with respect to the horizontal α= 2*θ+γ Will this relation hold true for all values of θ and γ ? Or do I need to modify the equation for different cases ? NOTE: I have used the normal x-y coordinate system and not oriented along the incline.