PROGRAM free_fall            ! no air resistance
CALL initial(y,v,a,g,t,dt)   ! initial conditions and parameters
CALL print_table(y,v,a,t,nshow)   ! print initial conditions
LET counter = 0
DO while y >= 0
   CALL Euler(y,v,a,g,t,dt)
   LET counter = counter + 1
   IF mod(counter,nshow) = 0 then
      CALL print_table(y,v,a,t,nshow)
   END IF
LOOP
CALL print_table(y,v,a,t,nshow)   ! print values at surface
END

SUB initial(y,v,a,g,t,dt)
    LET t = 0                ! initial time (sec)
    LET y = 10               ! initial height (m)
    LET v = 0                ! initial velocity
    LET g = 9.8              ! (magnitude) of accel due to gravity
    LET a = -g
    INPUT prompt "time step dt = ": dt
END SUB

SUB Euler(y,v,a,g,t,dt)
    LET y = y + v*dt         ! use velocity at beginning of interval
    ! following included to remind us that acceleration is constant
    LET a = -g               ! y positive upward
    LET v = v + a*dt
    LET t = t + dt
END SUB

SUB print_table(y,v,a,t,nshow)
    IF t = 0 then
       INPUT prompt "number of time steps between output = ": nshow
       CLEAR       ! might want to omit this graphics statement
       PRINT "time (s)", "y (m)", "velocity (m/s)", "accel (m/s^2)"
       PRINT
    END IF
    PRINT t,y,v,a
END SUB