Getting Started with Spline Fit in Matlab

[Arman777]

Hi Everyone

I am Arman.I am at first grade in Physics and I have trouble to understand Matlab.Now we did Energy Conservation Experiment.I draw x-t and F-t graphs using matlab.Instructor told us we should use spline fit to draw a F-t graph.I don't have a toolbox about it.She told us some code like

"hh=(xx,...)"

It was something like this I don't remember exactly.How can I do that.I can send my graph codes If its necessery.

Sincerely Arman

 

[DrClaude]

Have a look at http://www.mathworks.com/help/matlab/ref/spline.html

 

[Arman777]

Thank you. But I have some trouble again.

 

[DrClaude]

You don't have the same number of values of x as you have of F.

 

[Arman777]

I have again problem.

 

[DrClaude]

Why are you trying to spline F with respect to x? In the OP, you say you are working with F vs t.

 

[Arman777]

I should draw Fx so so sorry

 

[Arman777]

My foolish mistake If you can help me I ll be glad.

 

[DrClaude]

You mean that you plot force vs position, and you need to fit that curve? If that is the case, are all the values of x distinct?

 

[Arman777]

I have f-t and x-t grpahs and I want to make F-x.But When we want to make to grapsh together there will be a time difference.I mean let's suppose x-t graph is t=0.34 x=1.5 but in F-t graph t=0.35 F=1.5 so the time difference should be fixed and spline fit makes that thing I guess.

 

[Arman777]

.Basically I want to plot F-x Graph using F-t and x-t graph and spline fit

 

[Arman777]

If I can't I will going to just take F and x and put them.Which not correct put Its fine enough ??

 

[DrClaude]

Then what you want to do is to spline F at the same points in time as x.

FF = spline(t,F,xx)

 

[Arman777]

It worked but graph is so nonsense.I don't know why ?

 

[Arman777]

what will be that plot code
"yy = spline(x,y,xx);
plot(x,y,'o',xx,yy)"

Example in the Mathlab

My idea
FF = spline(t,F,xx)
plot(t,F,'o',xx,FF)

 

[DrClaude]

plot(t,F,'o',xx,FF)

That will give you F vs t, not F vs x. Now that you have FF at the same points in time as x, you can plot
plot(x,F)

 

[Arman777]

Theres one problem again
1)I don't have 400 N but it still writes 400 N I don't know why ?

By the way thanks

 

[DrClaude]

Could you show the plot
plot(t,F,'o',xx,FF)

 

[Arman777]

ok

 

[DrClaude]

It doesn't work because you are extrapolating data outside the time range of the force measurements. I don't understand how come you have position data at such different times than force data.

 

[Arman777]

Let me send my data graphs each of them.And Data.If you want you can look.

 

[Arman777]

>> t=position_time(:,1);
>> x=position_time(:,2);
>> t=force_time(:,1);
>> F=force_time(:,2);
>> tt=0.8:3.1;
>> tt=0.8:0.1:3.1
FF=spline(t,F,tt);
>> plot(x,F);

It worked I can send the image its consist with the result.Thank you

 

[Arman777]

Is it consist with the two graph that I send you ?
And I have a problem My graph says the work done is 0.26 J using trapz code.
I calculated the ΔKE=1/2mΔv² Which gave me 0.42 J
and the real result using mgsinθx=W gave me 0.34 J

It means theoritaical value is 0.34 J , my experimental graph value is 0.26 J , experimental speed value is 0.42 J

There will be a friction and ΔKE-W_f=0.26J so 0.16 J gone to friction maybe ??

But there's also friction that we didnt reduce from experimental value which I thing that means I found the result with %10-15 error

Is that ok ?