Projectile Motion using ode45 in Matlab - matlab

Question

I'm trying to model projectile motion with drag in Matlab. Everything works perfectly....except I can't figure out how to get it to stop when the "bullet" hits the ground.
I initially tried an iteration loop, defining a data array, and emptying cells of that array for when the y value was negative....unfortunately the ode solver didn't like that too much.
Here is my code
function [ time , x_position , y_position ] = shell_flight_simulator(m,D,Ve,Cd,ElAng)
rho=1.2; % kg/m^3
g=9.84; % acceleration due to gravity
A = pi.*(D./2).^2; % m^2, shells cross-sectional area (area of circle)
function [lookfor,stop,direction] = linevent(t,y);
% stop projectile when it hits the ground
lookfor = y(1); %Sets this to 0
stop = 1; %Stop when event is located
direction = -1; %Specify downward direction
options = odeset('Events',#event_function); % allows me to stop integration at an event
function fvec = projectile_forces(x,y)
vx=y(2);
vy=y(4);
v=sqrt(vx^2+vy^2);
Fd=1/2 * rho * v^2 * Cd * A;
fvec(1) = y(2);
fvec(2) = -Fd*vx/v/m;
fvec(3) = y(4);
fvec(4) = -g -Fd*vy/v/m;
fvec=fvec.';
end
tspan=[0, 90]; % time interval of interest
y0(1)=0; % initial x position
y0(2)=Ve*cos(ElAng); % vx
y0(3)=0; % initial y position
y0(4)=Ve*sin(ElAng); % vy
% using matlab solver
[t,ysol] = ode45(#projectile_forces, tspan, y0);
end
end
x = ysol(:,1);
vx = ysol(:,2);
y = ysol(:,3);
vy = ysol(:,4);
plot(x,y, 'r-');
xlabel('X Position (m)');
ylabel('Y Position (m)');
title ('Position Over Time');
end
I thought this would define an event when y=0 and stop the projectile, but it doesn't do anything. What am I doing wrong?

Answer 1

When trying to find the time at which the solution to the ODE reaches a certain level you should use an
Events function - see the BALLODE demo for an example that stops the solution process when one of the components of the solution reaches 0.