use of matlab in physics

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Tor Fredrik Hove
Tor Fredrik Hove 2012-2-3
You have a force from many forces that is
F=F+Fc-Fv-D
where
F=400 N
Fc=488*exp(-(t/0.67)^2) N
Fv=25.8v
D=0.34911(1-0.25*exp(-(t/0.67)^2)*v^2
i was asked to use eulers method to calculate in matlab the position of a runner who is moving with these forces (F is moving forward motion, Fc is extra motion forward when body is leaned and back is curled, Fv is a force that prevents F to continue to infinity and decreases acceleration as function of v of runner and D is airdrag)
here is what i wrote but without clearly getting a right answer:
vl=[];
x=[];
t=[];
a=[];
vl(1)=0;
n=2;
x(1)=0;
t(1)=0;
dt=0.05;
while x(n-1)<100
F=5;
Fc=6.1*exp(-(t(n-1)/0.67)^2);
Fv=25.8*vl(n-1);
D=0.34911*(1-0.25*exp(-(t(n-1)/0.67)^2))*(vl(n-1))^2;
a(n-1)=F+Fc-Fv-D
vl(n)=vl(n-1)+a(n-1)*dt;
x(n)=x(n-1)+vl(n)*dt;
t(n)=t(n-1)+dt;
n=n+1;
end
plot(t,vl,'r');
hold on
plot(t,x,'g');
legend('vl','x');

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采纳的回答

the cyclist
the cyclist 2012-2-3
I notice two things:
  1. You run your while loop out to x=100 or so, but with the constants you have, all the interesting action is taking place at x<1. Temporarily, you might consider changing the limit to x(n-1)<1, while you debug.
  2. You divided two of your force constants (F and Fc) by a factor of 80, but you did not do the same with the other two (Fv and D). I did not look in detail (something with units, which you did not specify?), but that might lead to the current incorrect behavior, where the runner reaches top velocity in about a second.
I suggest you write out your equations with the units included, to ensure that the units all match.
There are also some significant MATLAB-specific improvements that could be made to your code, but I think you should focus on getting the physics/math correct first.

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