Question

Let there be two balls, one of which is moving about in the Cartesian coordinate plane, while the other is stationary and immobile. At some point, the moving ball collides with the inert ball. Assuming the moving ball is traveling in a straight line, how can one derive the new angle that the moving ball will be propelled given the following information:

The moving ball’s center coordinates (X0, Y0), radius (R0), and angle of travel before impact (A0)

The stationary ball’s center coordinates (X1, Y1) and radius (R1)

Answer 1

If your second ball has infinite mass:

Where phi (after a long calc) is:

phi=  -ArcTan[
         ( 2 R^2 Sin[A0] + 2 (YD Cos[A0] - XD Sin[A0]) (2 H Cos[A0] + 
           2 XD Sin[A0]^2 - YD Sin[2 A0]))  /
         ((2 R^2 - XD^2 - 3 YD^2) Cos[A0] + (XD^2 - YD^2) Cos[3 A0] + 
           8 XD YD Cos[A0]^2 Sin[A0] + 4 H Sin[A0] (-YD Cos[A0] + XD Sin[A0]))
           ]

Where:

H   = (R0 + R1)^2 - ((Y0 - Y1) Cos[A0] + (X0 - X1) Sin[A0])^2  
R^2 = (R0 + R1)^2
XD  =  X1 - X0
YD  =  Y1 - Y0

Edit

To determine the whole trajectory, you’ll also need the coordinates for the center of the moving ball at the time of impact. They are:

  {X,Y}= {X1+Sin[A0] ((Y1-Y0) Cos[A0]+ (X0-X1) Sin[A0])-Cos[A0] Sqrt[H],
          Y1+Cos[A0] ((Y0-Y1) Cos[A0]+(-X0+X1) Sin[A0])-Sin[A0] Sqrt[H]}