How to plot a derivative against variable?

Question

River 2023-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/1939194-how-to-plot-a-derivative-against-variable

回答： Peter Perkins 2023-4-5

Use if statements to select an equation for x based on the inputed value for damping. Plot a graph of x (displacement) against time (this bit works). Then want to differentiate x for velocity, and differentiate velocity for acceleration and then plot all three. I used:

syms t

v = diff(x,1)

plot(t,v)

Get error "Data must be numeric, datetime, duration, categorical, or an array convertible to double."

x is a previously selected equation with t as a variable, also includes Z but this is asked for and inputted at the beginning.

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John D'Errico 2023-4-1

在 MATLAB Online 中打开

What do you expect? Read what you did. You did this:

syms t
v = diff(x,1)

What is x? Should MATLAB know that x is something possibly related to t? Why should it? Possibly x is a variable that already exists, since if it did not exist, you would have gotten a different error.

You claim that x already exists, but we don't see what it is. Is x an "equation", perhaps a function? Is it just a variable of some ilk? Is x something symbolic?

You are not telling us enough information to know what you did, except that you did something that caused an error.

River 2023-4-1

编辑：Walter Roberson 2023-4-1

在 MATLAB Online 中打开

%% Program to simulate the system by giving graphs for displacement, velocity and acceleration with respect to time.
% Selects equation and calculates displacement based on the given damping
% ratio.
clear variables
% Ask what the damping ratio is
Z = input('What is the damping ratio Z?');
t = (0.0:0.001:5);
% Equation for undamped
if Z == 0
    x = (0.01.*cos(595.6.*t)) + (0.1599.*cos((590.*t)-0.3886));
    % Equation for underdamped
elseif (0 < Z) && (Z<1)
    x =(exp(-595.60.*Z.*t).*((0.01.*cos(595.6.*t.*sqrt(1-(Z^2)))) + (((0.01.*Z)./(sqrt(1-(Z^2)))).*sin(595.6.*t.*sqrt(1-(Z^2)))))) + (0.1599.*cos((590.*t)-0.3886));
    % Equation for critically damped
elseif Z == 1
    x = ((0.01 + 5.966.*t).*(exp(-595.6.*t))) + (0.1599.*cos((590.*t)-0.3886));
    % Equation for overdamped
elseif Z > 1
    x = (((0.0017 + Z + sqrt(Z^2 - 1))./(200*sqrt(Z^2 - 1)))*exp((-595.6*Z + 595.6*sqrt(Z^2 - 1))*t)) + ((0.01 - ((0.0017 + Z + sqrt(Z^2 - 1))./(200*sqrt(Z^2 - 1))))*exp((-595.6*Z + 595.6*sqrt(Z^2 - 1))*t)) + (0.1599.*cos((590.*t)-0.3886));
    % Ask for a positive number if a negative one is entered
else 
    disp('Please enter a positive value for Z.')
end
plot(t,x,'b')
grid on
grid minor
xlabel('Time (s)')
ylabel('Displacement (m)')
title('Displacement of mass in micro-energy harvester')
% Differentiate displacement for velocity and display a graph agaist time.
syms t
v = diff(x,1);
plot(t,v)