Help with my 2nd Order ODE Solver.

A penny dropped from the top of a building (138meters). Using ode45 compute the motion of the penny’s fall. Which I have done below. I just need the function to stop when the penny hits the ground. I can guess the penny hits roughly at 13.25 seconds from the graph and math. In my second funtion I am trying to make it where my function stops when the pennny hits the goround(y=0). Plz help any suggestions will be helpful.

yo = 138;

yf = 0;

a = 9.8;

to = 0;

tf = 20; making the final time longer than need be becuse I want to make my event fcn to work.

time = [to tf];

iv = [138 0];

Options = odeset('RelTol',100*eps,'Events',@ground);

[t,y,te,ue,ie] = ode45(@f,time,iv,Options);

%%%FUNTIONS%%%

function rk=f(t,y)

ag = 9.8;

vt = 11;

rk = [y(2); (-ag)*(1-(y(2)/vt)^2)] % this is my system of equations

return

end

function [value, isterminal, direction] = ground(t,y)

gd = 138;

value = y(2)-gd;

isterminal = 1;

direction = -1;

end

Answer 1

[Star Strider]

Trigger the event on ‘y(1)’, and set ‘gd’ to zero:

function [value, isterminal, direction] = ground(t,y)
    gd = 0;
    value = y(1)-gd;   
    isterminal = 1;   
    direction = -1;   
end

That worked as I suspect it should, stopping the integration at about 13.3 seconds simulation time.

[Drake]

Thank you very much. :)

[Star Strider]

As always, my pleasure!