Manipulate[
 Grid[{{
    Show[
     Graphics[{
       Text[Style["+", Large], {25, d/2}],
       Text[Style["-", Large], {25, -d/2}],
       Arrow[{{-2, 0}, {0, 0}}],
       Line[{{0, d/2}, {22, d /2}}],
       Line[{{0, -d /2}, {22, -d /2}}]
       }],
     d1 = d;
     ParametricPlot[{v*t*Cos[a],
       Piecewise[{
         {0, t < 0},
         {If[
           c == "-", (v*t*Sin[a]) + U q/(2 d m) (t)^2, (v*t*Sin[a]) -
            U q/(2 d m)*(t )^2], t > 0}}]}, {t,
       0, (v^2*Sin[2*a]/ (U q/(d m )))},
      PlotStyle -> {Red, Dashing[Small]}
      ],
     Axes -> {True, True},
     AxesOrigin -> {0, 0},
     PlotRange -> {{-25, 25}, {-d/2, d/2}},
     AspectRatio -> 0.5,
     ImageSize -> 1.3 {400, 200}]},
   {Text@Grid[{
       {"Jačina el. polja " , "=", NumberForm[N[ U / d], {4, 2}] ,
        "V/m" },
       {"Domet " , "=",
        NumberForm[N[ v^2*Sin[2*a]/ (U q/(d m ))], {4, 2}], "m"}},
      Alignment -> Right]
    }}],
 {{d, 2, "Širina kondenzatora (m)"}, 1, 5, Appearance -> {"Labeled"}},
 {{U, 50, "Napon na kondenzatoru (V)"}, 10, 100,
  Appearance -> "Labeled"},
 {{v, 5, "Početna brzina čestice (m/s)"}, 1, 10,
  Appearance -> "Labeled"},
 {{m, .5, "Masa čestice (kg)"}, .1, 1, Appearance -> "Labeled"},
 {{q, 5, "Naelektrisanje čestice (C)"}, 1, 10,
  Appearance -> "Labeled"},
 {{a, Pi/3, "Upadni ugao"}, 0, 6.28, Appearance -> "Labeled"},
 {{c, "+", "Znak naelektrisanja"}, {"+", "-"}}, ControlPlacement -> Top
 ]