# a simple direction field ODE
# xpp treats time differently from other variables
# and direction fields need two dimensions, so 
# we add dummy, s, which is just t + C
# here is the first example from class
# this is a function
f(v,s)=g-gamma*v/m
# 
# here are the ODEs
#
v'=f(v,s)
s'=1
# 
# here are some parameters
par g=9.8,gamma=2,m=10
#
# these commands control the plot range and the view
# they can also be changed within XPP
#
@ xlo=-1,xhi=10,ylo=45,yhi=60
@ xp=s,yp=v
done