Given function that integrates the pendulum nonlinear differential equation, plot the graph with the given conditions

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Mark Chernyshov
Mark Chernyshov 2018-10-9
评论： Walter Roberson 2018-10-11
Use the MATLAB function that integrates the pendulum nonlinear differential equation to find the trajectory of a pendulum of length 1m, with an initial displacement of pi/2 rad and -pi rad/s initial velocity. Integrate between 0 and 10s. Save your shot as PNG file.
function pendulum (1, x0, T)
% PENDULUM Computes trajectory of pendulum
% PENDULUM (1, x0, T)
% 1 - the length of the pendulum
% x0 - the comlumn vector of initial conditions:
% angular displacement and velocity
% T - the end time
g = 9.81; % m/s^2
options = odeset('MaxStep', 0.01, 'Stats', 'on');
sol = ode45(@(t, x) f(t, x, g, l), [0 T], x0, options);
subplot(2, 1, 1)
plot(sol.x, sol.y)
legend('angular displacement (rad)', ...
'angular velocity (rad/s)', ...
'Location', 'southwest')
title('waveforms')
xlabel('time (s)')
subplot(2, 1, 2)
plot(sol.y(1,:), sol.y(2,:))
title('phase plane')
xlable('angular displacement (rad)')
ylable('angular velocity (rad/s)')
end
function xdot = f(~, x, g, l)
% F pendulum differential equation
% x(1) is the angular displacement from the vertical
% x(2) is the angular velocity
% g is the acceleration of gravity
% l is the length of pendulum
xdot = [
x(2);
-g/l * sin(x(1))
];
end

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采纳的回答

Walter Roberson
Walter Roberson 2018-10-9
You cannot have a number as a parameter in a function statement.
We recommend against using lowercase L as the name of a variable, as it is easy to confuse it with the digit 1

更多回答（1 个）

Mark Chernyshov
Mark Chernyshov 2018-10-9
I need to plot this function, but the statement pendulum(1, [pi/2: -pi], 10) did not seem to do the plotting
  1 个评论
Walter Roberson
Walter Roberson 2018-10-11
According to the documentation, the second parameter needs to be
% x0 - the comlumn vector of initial conditions:
% angular displacement and velocity
Your parameter [pi/2: -pi], is a pair of numbers with the colon operator between them. The colon operator with two operands creates a vector starting from the first value and increasing by 1 each time until the second operand is reached or exceeded. When the second operand is less than the first, the colon operator returns the empty vector. Since -pi is less than pi/2, you are passing the empty vector.
Perhaps you mean [pi/2; -pi] . That would correspond to an angular displacement of pi/2 and a velocity of -pi . Seeing pi in a velocity looks odd, but it is not impossible.

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