How to multiply matrices with variables as elements

Question

Yau Ming Ng 2019-9-13

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https://ww2.mathworks.cn/matlabcentral/answers/480303-how-to-multiply-matrices-with-variables-as-elements

回答： Naveen Venkata Krishnan 2019-10-9

% v is the velocity of the projectile [1:60]
% theta is the angle of at which it is launched [0:75]
% dx is the distance of x-coordinate of the object from the origin after t seconds
% dy is the distance of x-coordinate of the object from the origin after t seconds
% Matrix 1, M1 = [0, v*cosd(theta); -4.9, v*sind(theta)]
% Matrix 2, M2 = [t^2; t]
% Matrix 3, M3 = [dx; dy]
[0, v*cosd(theta); -4.9, v*sind(theta)]*[t^2; t] = [dx; dy]

The formula above is the equation for travel of a projectile in matrix form.

I was planning on searching for the shortest time possible for an object to travel dx and dy (which I set)

My plan was to multiply the equation by the inverse of M1. And then find the minimum value of t using a min function, then obtaining its v and theta that way.

 [t^2; t]=1/(4.9*v*cos(theta))*[v.*sind(theta), -v.*cosd(theta); 4.9, 0]*[dx; dy]

In Matlab it would be closer to

[x; y]=1/(4.9*v*cos(theta))*[v.*sind(theta), -v.*cosd(theta); 4.9, 0]*[dx; dy]

and then finding the min for y.

However the variables caused the matrix to explode into a 2x(way too much) matrix.

> Error using vertcat
> Dimensions of arrays being concatenated are not consistent.

Is there another way of doing this?

Edit :

Disclaimer : If the variables look suspiciously familiar, my school requested that I pick v and theta by hand without caring for time taken

This was just a personal challenge using the work they gave me.

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Bob Thompson 2019-9-13

I am not well versed in these forms of matlab, but I know that symbolic values and function (f(x) math functions, not matlab script fuctions) do exist. I think you would have a better time looking down that route.

I believe you're receiving the error because v and theta are already defined arrays (they have sizes), but do not have the same size for their arrays, so it is difficult to multiply 60x1 elements of v with the 75x1 elements of theta.

Walter Roberson 2019-9-13

在 MATLAB Online 中打开

Caution:

[x; y]=1/(4.9*v*cos(theta))*[v.*sind(theta), -v.*cosd(theta); 4.9, 0]*[dx; dy]

your cos(theta) should be a cosd(theta)

And you would need to assign this to a single varible and extract the parts of the variable afterwards.

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Answer 1

Naveen Venkata Krishnan 2019-10-9

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Hello,

Based on the problem description you are trying to minimize the time taken to reach a particular set point ( cost function ) where Velocity(v) and Theta(θ) are the optimization variables . Assuming the vectors [1:60] and [0:75] denotes the Range of optimization variables ( v and θ), in that case

v-lower bound : 1;

v-upper bound :60;

θ-lowerbound : 0;

θ-upper bound : 75;

This can solved as an optimization problem using fmincon solver.

cost function :

where here x,y are dx,dy in your case.