Plot parabola with start, end and vertex points

Hi,

So I have three points and I would like matlab to plot them as a parabola. The points represent the path of a balls projectile motion and I know the start (0,0), end (2.062, 0) and high point (1.031, 0,73) of the parabola.

What I would like most would be to plot these points as a parabola and also calculate the equation for the parabola.

Thanks in advance :)

Freyja

Answer 1

[Roger Stafford]

Another way to get the parabola is to write:

 y = 0.73-a*(x-1.031)^2

which is the form it must have to peak at (1.031,0,73) and then set x and y to zero to force the parabola to contain (0,0) and solve for a. The solution is obvious. Then do a plot.

[Freyja]

Ok, well I worked out the value for a and then wrote a function, I'm sure matlab could have worked it out for me but I don't know how. So I did the easy way and plotted the range I wanted and it gave me a parabola!

So thanks a lot for the help. This is the code that worked for me...

x = (0:0.01:2.06);

y = 0.73-0.68676.*(x-1.031).^2;

plot(x,y,'LineWidth',3,'color',[1,0.5,0])

Many thanks!

Answer 2

[Matt J]

You can use POLYFIT to fit a parabola to the points. Then you can use POLYVAL to sample and plot it.

[Freyja]

Sorry, but I am new to the polyfit and polyval functions and I don't really understand how to use it in this context...can you give me an example?

[Freyja]

i tried this:

x = [0 1.03 2.06]; y = [0 0.73 0]; p = polyfit(x,y,2);

n = polyval(p,x);

plot(x,n)

But this gave me a triangle....not a parabola....

[Matt J]

But this gave me a triangle....not a parabola....

It only looks like a triangle, because you didn't plot at enough points

xu=linspace(0,2.06,1000);
plot(xu, polyval(p, xu));

[Freyja]

Oh I see, thankyou, i will try that :)