how can you plot a matrix row vs row using a for loop

We were given 5 springs and had to put 3 different masses on each spring and then measure the displacement for each spring for each mass (3 masses & 3 displacements per spring).

% Masses for each Spring [g]:          % 3 masses per spring
S1_M=[10 20 30];
S2_M=[20 30 50];
S3_M=[50 100 150];
S4_M=[100 200 300];
S5_M=[100 200 300];

% Displacement for each Spring [in]:        % Displacements for each mass
S1_D=[1.35 2.95 4.45];
S2_D=[1.05 1.65 2.85];
S3_D=[1.1 2.55 3.9];
S4_D=[1.75 4.1 6.3];
S5_D=[1.25 2.9 4.45];

% Convert displacement from [in] to [m]:
S1_D=S1_D*0.0254;
S2_D=S2_D*0.0254;
S3_D=S3_D*0.0254;
S4_D=S4_D*0.0254;
S5_D=S5_D*0.0254;

% Convert mass[g] to force[N]:
S1_M=S1_M*0.0098;
S2_M=S2_M*0.0098;
S3_M=S3_M*0.0098;
S4_M=S4_M*0.0098;
S5_M=S5_M*0.0098;

% Create a single matrix for all 5 springs:
Disp=[S1_M;S1_D;S2_M;S2_D;S3_M;S3_D;S4_M;S4_D;S5_M;S5_D];

I know im right up until this point.

Then it says: in a function, use polyfit to determine the stiffness of each individual spring. Use a for loop within the function to accomplish these caculations. There are 5 springs and we are supposed to plot the force versus the displacement for each individual spring in a new figure. Below is how I started it, am I even doing this right?

? - means I don't know what variable to put there

 % Create a for loop using polyfit to calculate the spring constants:
function[?]=?

[r,c]=size(Disp)
for i=1:1:r
  for j=1:1:c                                  %im missing the ployfit part
      plot(Disp(?),Disp(?),'bo')                in the for loop
  end    
end

[Will]

Can anybody help??? this is due today!!!!! Thanks

[Walter Roberson]

I do not seem to be up to concentrating on this at the moment. I think I need a nap.

[Daniel Shub]

I am closing this question because it treats disrespectful to the contributors to MATLAB answers. Please edit the question to fully reflect exactly what you have tried, where you are stuck, and a concrete answerable question. Please also show respect to the community.

[Will]

im sorry Daniel, as stated before i only saw that answering questions and i was trying to get their attention and i need help ASAP

my apologizes

[Daniel Shub]

http://www.mathworks.com/matlabcentral/answers/29922-why-your-question-is-not-urgent-or-an-emergency

You are essentially asking I to do your homework for you. That is not appreciated. Edit the question to show what you have tried, otherwise I will keep closing the question.

[Will]

Im sorry Daniel and Walter. I didnt know the policy and didnt mean to offend anyone. that was not my intentions at all. im stuck and dont know how to move on form here. i want nobody to do this for me. i just need help using polyfit to plot row vs row

sorry my my actions offending yal any help truly is greatly appreciated

Answer 1

[Image Analyst]

Assuming you have one mass at a time hanging vertically down, where the other end of the spring is attached to the ceiling, you can say force = k*distance = mass*acceleration = mass*g. So k = force / distance. You have the force and you have the distance so you just need to make up the 3 k's I would guess. Though I don't know why you need polyfit instead of just averaging the 3 k's. Then you do that for each of the 5 springs.

[Image Analyst]

Sorry - Just noticed your new subject line. I can take down my answer if you want. We all know the answer you'll get from Walter on your urgent request. Wait for it.....wait for it......

[Will]

we have to use ployfit to determine the slope. the slope that the polyfit gives you is the k value. we cant just hard code the calculation for k in our program

[Will]

I just said walter or azzi trying to get their attention because they were the only ones i saw answering questions and i have to finish this today. Any help is much needed and greatly appreciated

[Image Analyst]

So why can't you just do

coeffs = polyfit([1,2,3], [k1, k2, k3], 0);

[Sean de Wolski]

I would disagree with this statement:

I know im right up until this point.

Since the way you have defined variables is not at all conducive to for-loops.

FAQs

[Will]

the assignment says we have to use polyfit to get the k values and use a for loop to solve for these calculations

[Sean de Wolski]

Okay, this uses polyfit and a for-loop.

for ii = 1
  coeffs = polyfit([1,2,3], [k1, k2, k3], 0);
end

[Will]

Sean, how should i correct it then. I had to do all those calculations to get the correct units for my final matrix. the directions say we a have to have one matrix with all five springs in it. and use use that matrix to plot force vs mass foe each spring and use polyfit to calculate the spring constant. the plot and polyfit have to be contained in the for loop

[Sean de Wolski]

Well that's the problem.. You can't easily or normally loop over different variables:

SM=[10 20 30;20 30 50;50 100 150]; %etc
SM.*0.98

etc

Now you can loop over rows of this:

for ii = 1:size(SM,1);
   k(ii) = do_something_with(SM(ii,:));
end

[Will]

my final matrix is a 10x3:

Disp=[S1_M
      S1_D                             %Its a 10x3 beacuae we measured
      S2_M                              3 masses per spring
      S2_D
      S3_M                %I need to plot: row 1 vs 2
      S3_D                                 row 3 vs 4
      S4_M                                 row 5 vs 6 etc
      S4_D
      S5_M
      S5_D]

[Image Analyst]

Why don't you create 5 row by 3 column arrays for mass and displacement, where the 5 rows represent the different springs, and the 3 columns represent the different weights hanging on the spring? Then a loop would be much more appropriate.

[Will]

SP=spring M=mass(Force) D=Displacement

the problem says the format of the matrix has to be:

SP1-M1 SP1-M2 SP1-M3
SP1-D1 SP1-D2 SP1-D3
SP2-M1 SP2-M2 SP2-M3
SP2-D1 SP2-D2 SP2-D3
etc for each spring                 %i need to plot row 1vs2,3vs4,5vs6,etc

[Image Analyst]

So it's been a few days. I assume it's done by now, right?