V = Manipulate[
  h[x_] := x;
  a = 0;
  b = 1;
  startpos[k_] := a + (k - 1) 0.01;
  a1 = Plot[{1, -1, t, -t}, {t, 0, 1}, AspectRatio -> Automatic, 
    ImageSize -> Medium];
  b = Graphics[
    Table[{Red, Opacity[0.5], Disk[{x, 0}, {0.01, 2}], Green, 
      EdgeForm[Black], Disk[{x, 0}, {0.01, h[x]}]}, {x, 0, r, 0.1}], 
    AspectRatio -> Automatic, ImageSize -> Medium];
  aa = Table[
    Graphics3D[{{Opacity[2/5], 
       Cylinder[{{-1, 0, 0}, {1, 0, 0}}, 1]}, {Opacity[1/5], 
       Cylinder[{{i, 0, 0}, {i + 0.02, 0, 0}}, i]}}, Boxed -> False, 
     AspectRatio -> Automatic, ImageSize -> Medium, FaceGrids -> All, 
     ViewPoint -> Front], {i, 0, r, 0.1}];
  {Show[a1, b], Show[aa]}
  , {r, 0, 1}]