I really want to draw the precession of mercury

Question

Asir Tushar 2017-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/366241-i-really-want-to-draw-the-precession-of-mercury

评论： Meindert Norg 2018-1-9

clc;clear all;
a=10; 
b=5; 
lambda=.5; 
v1=1/20; 
v2=3; 
t=0:.1:40; 
x(t)=a.^2+b.^2 +a*cos(2*pi*v1*t)*cos(2*pi*v2*t)-lambda*b*sin(2*pi*v1*t)*sin(2*pi*v2*t); 
y(t)=a.^2+b.^2 +a*sin(2*pi*v1*t)*cos(2*pi*v2*t)-lambda.*b*cos(2*pi*v1*t)*sin(2*pi*v2*t);
plot(x(t),y(t))

This is my program. but it's not running cause there is a dimension mismatch. can anybody tell me what did i do wrong?

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Meindert Norg 2018-1-9

在 MATLAB Online 中打开

In case you have not figured it out in the mean time:

Remove the element-wise ".^2"

a^2+b^2

add element wise multiplication for the vectors where 't' is used:

a*cos(2*pi*v1*t).*cos(2*pi*v2*t)

fix the way you plot it:

plot(x,y)

Here is the result:

 clc;clear all;
 a=10; 
 b=5; 
 lambda=.5; 
 v1=1/20; 
 v2=3; 
 t=0:.1:40; 
 x=a^2+b^2 + a*cos(2*pi*v1*t).*cos(2*pi*v2*t)-
 lambda*b*sin(2*pi*v1*t).*sin(2*pi*v2*t); 
 y=a^2+b^2 + a*sin(2*pi*v1*t).*cos(2*pi*v2*t)-
 lambda.*b*cos(2*pi*v1*t).*sin(2*pi*v2*t);
 plot(x,y)

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Answer 1

Guillaume 2017-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/366241-i-really-want-to-draw-the-precession-of-mercury#answer_290418

在 MATLAB Online 中打开

As the error message says

x(t)

is only valid if t is a real positive integer.

The simplest way to solve your problem is to get rid of the indexing of x and y which is completely meaningless

x = a^2 + b^2 + a*cos(2*pi*v1*t) .* cos(2*pi*v2*t) - ...
y = a^2 + b^2 + ...
plot(x, y)

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Asir Tushar 2017-11-10

Thank you man.

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Answer 2

M 2017-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/366241-i-really-want-to-draw-the-precession-of-mercury#answer_290365

编辑：M 2017-11-10

在 MATLAB Online 中打开

First :

a.^2

the dot is useless as a is a scalar

but I think you should add a dot between the cos multiplication :

a*cos(2*pi*v1*t).*cos(2*pi*v2*t)

otherwise you will get an error.

Then, if you start at t=0, you will get an error as you're trying to acces

x(t)=x(0)

but Matlab indices should be positive integers.

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Asir Tushar 2017-11-10

在 MATLAB Online 中打开

x(t)=a^2+b^2 +a*cos(2*pi*v1*t).*cos(2*pi*v2*t)-lambda*b*sin(2*pi*v1*t).*sin(2*pi*v2*t); 
y(t)=a^2+b^2 +a*sin(2*pi*v1*t).*cos(2*pi*v2*t)-lambda*b*cos(2*pi*v1*t).*sin(2*pi*v2*t);

i corrected the code like you said. still this message is showing for these two lines "Subscript indices must either be real positive integers or logicals.

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