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I need to plot the velocity function versus time, but when I do, nothing shows up on the graph. Any ideas?
t = [0,0.52,1.04,1.75,2.37,3.25,3.83];
x = [153,185,208,249,261,271,273];
%plot approximate velocity over [0,3.83]
%part i, create x(t) then find derivative
i = interp1(x,t,0,'spline');
f = @(xx)interp1(t,x,xx,'spline');
velocity = derivf(f,0,0.0001); %88.2979
plot(velocity,[0 3.83],'g')
xlabel('time(s)')
ylabel('position(m)')
hold on

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Walter Roberson
Walter Roberson 2021-3-25
What is the code for your derivf function? It does not appear to be a Mathworks function.
It's one in my text book:
function [Df]=derivf(f,x,h)
h1=h;
h2=h/2;
Df1=(-f(x+2*h1)+8*f(x+h1)-8*f(x-h1)+f(x-2*h1))/(12*h1);
Df2=(-f(x+2*h2)+8*f(x+h2)-8*f(x-h2)+f(x-2*h2))/(12*h2);
Df=4/3*Df2-1/3*Df1;
end

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Walter Roberson
Walter Roberson 2021-3-25

0 个投票

velocity = derivf(f,0,0.0001); %88.2979
that passes the scalar with value 0 as the x to derivf
Df=4/3*Df2-1/3*Df1;
That calculates an output the same size as x .
You are calling it with x being the scalar 0. So velocity will be a scalar.
plot(velocity,[0 3.83],'g')
velocity is a scalar. When you plot with a scalar x, you get dots at each of the y values... but you do not have markers turned on so you will not even see the dots. You could
plot(velocity,[0 3.83],'*g')
to see the points (velocity,0) and (velocity, 3.83)
I suspect you were thinking that you were asking to plot velocity over a range of x values from 0 to 3.83, but you are not doing so. Automatic plotting like that needs fplot() instead of plot(), and it needs the first parameter to be a function handle or a symbolic expression or a symbolic function.

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