Hi,

If you just want to solve the equations, why not use the built-in NDSolve function?

Here is how I would do this:

lagrangian = 1/2 (x'[t]^2 + y'[t]^2) + 1/Sqrt[x[t]^2 + y[t]^2];
eq = Table[
  D[lagrangian, x1] - D[D[lagrangian, D[x1, t]], t] == 
   0, {x1, {x[t], y[t]}}];
sol = NDSolve[
  Join[eq, {x[0] == 1, x'[0] == 0, y[0] == 0, y'[0] == 1}], {x[t], 
   y[t]}, {t, 0, 5}];
ParametricPlot[{x[t], y[t]} /. sol[[1]], {t, 0, 5}]

It's another story if you want to write your own solver though.

With minor code fixes it works:

eulerStep[{t_, state_List}, h_, f_List] := {t + h, 
  state + h Through[f[{t, state}]]}
solveSystemEuler[{t0_, state0_}, h_, n_Integer, f_List] := 
 NestList[eulerStep[#, h, f] &, {t0, state0}, n]
midptStep[{t_, state_List}, h_, f_List] := {t + h, 
  state + h Through[
     f[{t + 1/2 h, state + 1/2 h Through[f[{t, state}]]}]]}
solveSytemMidPt[{t0_, state0_}, h_, n_Integer, f_List] := 
 NestList[midptStep[#, h, f] &, {t0, state0}, n]

L = 1/2 (x'[t]^2 + y'[t]^2) + 1/Sqrt[x[t]^2 + y[t]^2];
D[D[L, x'[t]], t] - D[L, x[t]] == 0
D[D[L, y'[t]], t] - D[L, y[t]] == 0

xdot[{t_, {x_, vx_, y_, vy_}}] := vx
vxdot[{t_, {x_, vx_, y_, vy_}}] := -x/(x^2 + y^2)^(3/2)
ydot[{t_, {x_, vx_, y_, vy_}}] := vy
vydot[{t_, {x_, vx_, y_, vy_}}] := -y/(x^2 + y^2)^(3/2)
start = {1, 0, 0, 1};
fcns = {xdot, vxdot, ydot, vydot};

orbit = solveSystemEuler[{0, start}, 0.01, 800, fcns];
xypts = orbit\[Transpose][[2]]\[Transpose][[{1, 3}]]\[Transpose];
ListPlot[xypts]

Regards OS

Oliver Seipel please help me:

MultipleListPlot[xypts, PlotJoined -> True, 
  SymbolShape -> None, PlotStyle -> {Hue[0], Hue[0.66]}, 
  AspectRatio -> Automatic];

Is my code correct? Please help.

thanks Oliver Seipel.

I want to look like this:

what is the right code?

earthorbit = xypts;

spacestart = {1, 0, 0, 1.25}; orbit = 
 solveSytemMidPt[{0, spacestart}, 0.01, 2200, fcns];
spacehiporbit = 
  orbit\[Transpose][[2]]\[Transpose][[{1, 3}]]\[Transpose];
ListPlot[spacehiporbit]

ListLinePlot[earthorbit, spacehiporbit, 
 PlotStyle -> {Hue[0], Hue[0.66]}, AspectRatio -> Automatic]

I tried it but did not get the graphics I wanted. Please help me..

Use {} around your data for ListLinePlot -- it expects a list of data (which are in the form of a list as well).

ListLinePlot[{earthorbit, spacehiporbit}, 
 PlotStyle -> {Hue[0], Hue[0.66]}, AspectRatio -> Automatic]

The right code to display 22 graphs. Like this:

The graph is obtained from the following old code:

Table[MultipleListPlot[Take[earthorbit, {n, n + 100}], 
Take[spaceshiporbit, {n, n + 100}], PlotJoined -> True, 
SymbolShape -> None, PlotStyle -> {Hue[0], Hue[0.66]}, 
PlotRange -> {{-3.6, 1.2}, {-2, 2}}, 
AspectRatio -> Automatic];, {n, 1, 2200, 100}];

then I change the code as follows:

Table[ListLinePlot[Take[earthorbit, {n, n + 100}], 
Take[spaceshiporbit, {n, n + 100}], 
PlotStyle -> {Hue[0], Hue[0.66]}, 
PlotRange -> {{-3.6, 1.2}, {-2, 2}}, 
AspectRatio -> Automatic];, {n, 1, 2200, 100}];

Running the program gave the following messages.

Is my code correct?

I tried it but the results of running the program are like this.

I want to show is a few points along a hundred-long space orbit. notice from space how to get a shorter trace when it's further: a description of Kepler's II law. Is my code correct? what's the problem?

I hope someone can help me. Thank You.