# Matlab Spline Fit

• MATLAB
Gold Member
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 dont have a toolbox about it.She told us some code like

"hh=(xx,....)"

It was something like this I dont remember exactly.How can I do that.I can send my graph codes If its necessery.

Sincerely Arman

Gold Member
Thank you. But I have some trouble again.

#### Attachments

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

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

Gold Member
I should draw Fx so so sorry

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

DrClaude
Mentor
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?

Gold Member
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 lets 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.

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

Gold Member
If I cant I will gonna just take F and x and put them.Which not correct put Its fine enough ??

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

FF = spline(t,F,xx)

• Arman777
Gold Member
It worked but graph is so nonsense.I dont know why ?

Gold Member
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
Mentor
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
DrClaude
Mentor
Could you show the plot
plot(t,F,'o',xx,FF)

DrClaude
Mentor
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.

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

#### Attachments

Gold Member
>> 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

Gold Member
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-Wf=0.26J so 0.16 J gone to friction maybe ??

But theres 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 ?

Last edited: