void freefall_h(){
 double x=100;//[m]
 double x0=0;//[m]
 double v=100;//[m/s]
 double v0=0;//[m/s]
 double t;//[s]
 double dt;//[s]
 double g=9.8;

 TGraph *gr_v = new TGraph(512*100);
 TGraph *gr_x = new TGraph(512*100);

 t=0;//iinuma 
 dt=1E-3;//iinuma 1msec
 int i=0;//iinuma
 while(t<5){
        t+=dt;

  double k[4], l[4];
	 k[0] = dt*v0;
	 l[0] = dt*(-g);
	 k[1] = dt*(v0+l[0]/2.0);
	 l[1] = dt*(-g);
	 k[2] = dt*(v0+l[1]/2.0);
	 l[2] = dt*(-g);
	 k[3] = dt*(v0+l[2]);
	 l[3] = dt*(-g);
 
	 x=x0+(k[0]+2*k[1]+2*k[2]+k[3])/6.0;
	 v=v0+(l[0]+2*l[1]+2*l[2]+l[3])/6.0;
        
         x0=x;
         v0=v;
         if(i<512*100)gr_v->SetPoint(i,t,v);
         if(i<512*100)gr_x->SetPoint(i,t,x);
  printf("point at t=%lf x=%lf \n",t,x);
         i++;
 }
//iinuma
///////Graph
gROOT->SetStyle("Plain");
gROOT->ForceStyle();
gStyle->SetOptStat(0);
gStyle->SetCanvasBorderMode(0);
gStyle->SetPadColor(0);
gStyle->SetTitleFillColor(0);
gStyle->SetStatColor(0);
gStyle->SetCanvasColor(0);
gStyle->SetFrameLineColor(1);


 TCanvas *c = new TCanvas("c","check x",200,100,500,500);
 c->SetLeftMargin(0.13);
 TH2F* frame = new TH2F("frame"," ",10, -0.1, 6, 100, -150, 10);
 frame->SetTitle("");
 frame->GetXaxis()->SetLabelSize(0.04);
 frame->GetXaxis()->SetTitleSize(0.05);
 frame->GetYaxis()->SetLabelSize(0.04);
 frame->GetYaxis()->SetTitleSize(0.05);
 frame->GetXaxis()->SetTitle("t[sec]");
 frame->GetYaxis()->SetTitle("x[m]");
 frame->GetYaxis()->SetTitleOffset(1.2);
 frame->SetStats(0);
 frame->Draw();
   gr_x->SetMarkerStyle(7);
   gr_x->SetMarkerColor(1);
   gr_x->SetLineWidth(2);
   gr_x->Draw("p");
 c->SetGrid();

}