program try_k
      real mass,k
      mass=0.254*0.001
      ymax=4.0

      i_dummy=pgopen('/xwin')
      call pgenv(0.0,1.0,0.0,4.0,0,0)
      open(unit=10,file='fall.dat')
      read(10,*)n
      do i=1,n
        read(10,*)t,y
        call pgpt(1,t,ymax-y,9)
      end do
      j_color=1

 1000 write(*,*)'k?'
      read(*,*)k
      y=ymax-0.075
      vy=-0.56818
      t=0.0
      dt=0.00001
      call pgsci(mod(j_color,15))    
      do while(y>0)
        t=t+dt
        ay=(-9.8) + (-1)*k*vy/mass
        vy=vy+ay*dt
        y=y+vy*dt
        call pgpt(1,t,y,-1)
        j_color=j_color+1
      end do
      goto 1000
      call pgclos
      end