MATLAB Help

1. Jan 17, 2006

morry

Ok, so Ive been given a curve in polar coordinates.

I came up with a parameterisation:
x(t)=rcos(theta)
y(t)=rsin(theta)

But now I have to plot the graph using matlab and I have no idea. Theta lies between 0 and 2pi.

This is what I put in and got back in matlab:
>> t=[0:pi/50:2pi]
??? t=[0:pi/50:2pi]
|
Error: Missing MATLAB operator.

Cheers.

2. Jan 17, 2006

Tide

Should that be 2*pi?

3. Jan 23, 2006

morry

Thanks Tide. I must still be doing something wrong.

The graph is archimedes spiral. r= theta

Heres what Ive tried to do:

>>syms x y r theta
>>x=r*cos(theta)
>>y=r*sin(theta)
>>ezpolar(x,y)

I only need to plot from theta= 0-->2pi

Im really lost.

Any info on how to do line integrals would also be appreciated.

Thanks again.

4. Jan 23, 2006

Tide

You need to find the element of arc length along the curve. This should help:

$$ds = \sqrt {dr^2 + r^2 d\theta^2}$$

which you can easily integrate.

5. Jan 24, 2006

morry

Thanks Tide. Im not having trouble doing the maths by hand. I can find line integrals no worries by hand its just that I have no idea how to use matlab.

6. Jan 24, 2006

Tide

morry,

Since $r = \theta$ you just need to evaluate the integral

$$\int_{0}^{2\pi} \sqrt{1+\theta^2} d\theta$$

so look for a MatLab command resembling "int(sqrt(1+x^2), x = 0..2pi)"

7. Jan 24, 2006

morry

Thanks for the help again Tide, I think Ive got it all out! Finally!