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About the error message Indeterminate

I think you all might have been check the example the bouncer in the
entry Dynamic. But it is on a flat ground. So I want to make some
change to let it bounce on hills, which formed by Sine function. The
collisional consummation and reflection angle have been taken
considered, but when the point touches the ground, it doesn't move any
longer and the velocity shows indeterminate. I know indeterminate
means such an issue like 0/0, but I replace my reflecting function
with the colliding position and get a certain answer. I don't know if
is there any other issues will cause indeterminate.

Anyway, check the code first.

function[x_] := Sin[x] + 0.5 Sin[6 x];
Reflection[{{x_, y_}, {vx_, vy_}}] := {{x, y},
  0.8 Sqrt[vx^2 + vy^2] {Cos[2 ArcTan[function'[x]] - ArcTan[vy/vx]],
    Sin[2 ArcTan[function'[x]] - ArcTan[vy/vx]]}}

(* where function[] means the ground and Reflection[] shows how the
ball bounces up, where x, y means position and vx, vy means velocity.
0.8 Is the consummation of collision, Sqrt is the norm of speed and
the latter stuff is the new velocity in x and y direction. *)

PointSet = {{4, 6}, {0, -0.01}};
Plot[function[x], {x, -5, 5}, Axes -> None, Filling -> Bottom,
 PlotRange -> {{-5, 5}, {-2, 8}}, AspectRatio -> 1,
 Epilog ->
    PointSet =
     If[PointSet[[1, 2]] >=
       function[PointSet[[1, 1]]], {PointSet[[1]] + PointSet[[2]],
       PointSet[[2]] + {0, -0.001}}, Reflection[PointSet]];

(*I used Epilog to draw the point. I didn't use Mouseclick because it
will cause a dump*)

I'll keep waiting here and thanks in advance.

Tao Yue

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