Bike model with and without air resistance using matlab

Hi I'm trying to follow along in a computational physics book (Computational Physics; Nicholas Giordano 2nd edition) that attempts to model a bike with and without air resistance. I did everything that the book did but I got a completely different plot at the end.This is my code for the bike with air resistance.

function bike(v0,dt,tf)

t = 0:dt:tf;

v(1) = v0;

P = 400;

m = 70;

p = 1.225

A = 0.33

for i = 1:length(t)-1

    v(i+1) = v(i) + (P/m*v(i)-(p*A*v(i)^2/2*m))*dt;
    t(i+1) = t(i) + dt;
end

plot(t,v,'b');

title('Comparison of Euler approximation to actual solution')

xlabel('time')

ylabel('v')

disp(v(end));

P stands for power, m is mass, p is density of air, and A is the frontal area of the rider.

This is my code for the bike without air resistance.

function bike(v0,dt,tf)

t = 0:dt:tf;

v(1) = v0;

P = 400;

m = 70;

for i = 1:length(t)-1

    v(i+1) = v(i) + (P/m*v(i))*dt;
    t(i+1) = t(i) + dt;
end

plot(t,v,'b');

title('Comparison of Euler approximation to actual solution')

xlabel('time')

ylabel('v')

disp(v(end));

If someone could please help me out, I would really appreciate it.

Answer 1

[pfb]

The dimensions in your formulas do not look right. I think it is a matter of operator precedence. 1/2*3 is not the same as 1/(2*3).

Also, note that you build the time vector twice: once at the beginning

 t = 0:dt:tf

and then in the loop

t(i+1) = t(i) + dt;

However this should be no problem.

[Mohamed Saeed]

Wow, you're right. Thanks a lot!