ode45 goes busy endlessly when using the sign of a state variable

Question

Diaa 2021-4-23

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/810590-ode45-goes-busy-endlessly-when-using-the-sign-of-a-state-variable

回答： Diaa 2021-4-23

采纳的回答： Diaa

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This term "- fr*Mass*9.8*sign(y(2))" makes ode45 busy forever specifically when using 'sign(y(2))'

I just need to make -fr*Mass*9.8=0 only when y(2)=0

Any explanations are highly welcomed.

clearvars
clc
close all
[t,y] = myexercise;
plot(t,y(:,2))
function [t,y] = myexercise
tspan = [0 , 10];
y0 = [ 0 ; 5 ];
Mass = 1300;
CD = 0.3;
fr = 0.015;
Af = 1.6;
traction = @(t) (t>3&t<5)*5*t;
[t,y] = ode45(@(t,y) deceleration(t,y) , tspan , y0);
    function yp = deceleration(t,y)
        yp = [ ...
            y(2) ;...
            ( -0.5*1.23*CD*Af*y(2)^2 + traction(t) - fr*Mass*9.8*sign(y(2)) )/Mass ... 
            ];
    end
end

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James Tursa 2021-4-23

Please explain the physical problem and post the differential equation you are trying to solve so we can understand why you think you need the sign function.

Diaa 2021-4-23

It is a moving vehicle subjected to traction force and resistive aerodynamic drag and rolling resistance. Your comment made me realize that I forgot dividing the whole second equation by Mass. After fixing it, fr*Mass*9.8)*(y(2)>0) was enough to make it work.

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