Matlab and Pendulum function - Need Help - Thank you.

Question

DJ V 2016-12-6

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https://ww2.mathworks.cn/matlabcentral/answers/315713-matlab-and-pendulum-function-need-help-thank-you

回答： Weronika Izdebska 2018-9-4

function T= pendulum( L, a0 )
%PENDULUM Summary of this function goes here
%   Detailed explanation goes here
%initializing variables
alpha = a0;
delta_t = 1*10^-6;
g = 9.8;
stop = false;
theta = a0;
t = 0;
omegalast =0
%Set stop = falsE so that loop stop isn't true at start
while stop == false;
%increment t for each step into loop
t = t+ delta_t;
%compute alpha using given formula
alpha = - g/L*sin(theta);
%compute omega as alpha times timestep, delta_t
omega = alpha*delta_t;
%theta is then theta plus omega times delta_t
theta = theta + omega*delta_t;
%check to see if pendulum speed has
%slowed down from the last timestep
%This indicatest that pendulum is now past the
%minimum point so stop program
if abs(omega) <abs(omegalast)
    stop = true;
end
%IF THE SOFTWARE SHOULD STOP IT IS NOW SET TO DO SO
%SET OMEGALAST EQUAL TO OMEGA FOR CHECK IN NEXT TIME STEP
omegalast = omega;
end
%COMPUTE PERIOD AS 4 TIMES ELAPSED TIME TO BOTTOM OF PENDULUM
T = 4*t;
end

This isn't producing the right answers,and I don't know why. Perhaps a physic or math major can help me?

The problem is stated as:

* Write a function called pendulum that is called like this: T = pendulum(L,a0), where all arguments are scalars and a0 is a positive number less than π. The function calculates the period T of a simple pendulum, which is the time required for a weight attached to a rod of length L and negligible weight to start from rest, swing with no friction under the influence of gravity from an initial angle a0, to – a0 and back to a0 again, as shown in the figure. The motion is determined by physics using the following definitions, where units [square brackets] are provided but are not needed: θ = angle [radians]

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DJ V 2016-12-6

在 MATLAB Online 中打开

Perhaps a little easier to read with indentation:

function T= pendulum( L, a0 )
    %PENDULUM Summary of this function goes here
    %   Detailed explanation goes here
    %initializing variables
    alpha = a0;
    delta_t = 1*10^-6;
    g = 9.8;
    stop = false;
    theta = a0;
    t = 0;
    omegalast =0
    %Set stop = false so that loop stop isn't true at start
    while stop == false;
        %increment t for each step into loop
        t = t+ delta_t;
        %compute alpha using given formula
        alpha = - g/L*sin(theta);
        %compute omega as alpha times timestep, delta_t
        omega = alpha*delta_t;
        %theta is then theta plus omega times delta_t
        theta = theta + omega*delta_t;
        %check to see if pendulum speed has
        %slowed down from the last timestep
        %This indicatest that pendulum is now past the
        %minimum point so stop program
        if abs(omega) <abs(omegalast)
            stop = true;
        end
        %IF THE SOFTWARE SHOULD STOP IT IS NOW SET TO DO SO
        %SET OMEGALAST EQUAL TO OMEGA FOR CHECK IN NEXT TIME STEP
        omegalast = omega;
    end
    %COMPUTE PERIOD AS 4 TIMES ELAPSED TIME TO BOTTOM OF PENDULUM
    T = 4*t;
end

DJ V 2016-12-6

编辑：DJ V 2016-12-6

DJ V 2016-12-6

在 MATLAB Online 中打开

CODE WITH INDENTS:

 function T= pendulum( L, a0 )
      %PENDULUM Summary of this function goes here
      %   Detailed explanation goes here
      %initializing variables
      alpha = a0;
      delta_t = 1*10^-6;
      g = 9.8;
      stop = false;
      theta = a0;
      t = 0;
      omegalast =0
      %Set stop = false so that loop stop isn't true at start
      while stop == false;
          %increment t for each step into loop
          t = t+ delta_t;
          %compute alpha using given formula
          alpha = - (g/L)*sin(theta);
          %compute omega as alpha times timestep, delta_t
          omega = alpha*delta_t;
          %theta is then theta plus omega times delta_t
          theta = theta + omega*delta_t;
          %check to see if pendulum speed has
          %slowed down from the last timestep
          %This indicatest that pendulum is now past the
          %minimum point so stop program
          if abs(omega) <abs(omegalast)
              stop = true;
          end
          %IF THE SOFTWARE SHOULD STOP IT IS NOW SET TO DO SO
          %SET OMEGALAST EQUAL TO OMEGA FOR CHECK IN NEXT TIME STEP
          omegalast = omega;
      end
      %COMPUTE PERIOD AS 4 TIMES ELAPSED TIME TO BOTTOM OF PENDULUM
      T = 4*t;
  end

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Answer 1

Geoff Hayes 2016-12-6

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链接

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https://ww2.mathworks.cn/matlabcentral/answers/315713-matlab-and-pendulum-function-need-help-thank-you#answer_246238

在 MATLAB Online 中打开

DJ - you've missed two important points in the problem description. The first is the

...the angular velocity omega is increased by the product of the angular acceleration and delta t..

This means that instead of

omega = alpha*delta_t;

you would initialize omega to zero (outside of your while loop since the pendulum is initially at rest) and then increase it (on each iteration) as

omega = omega + alpha*delta_t;

And the algorithm

...continues until the pendulum has passed its lowest point, at which theta equals zero...

So you would break out of your while loop once theta is near or less than zero.

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DJ V 2016-12-6

That was it. Thanks a lot!

Souvik singha 2017-6-11

please sir if you can showcase your final and correct code here, it would be very helpful.

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Answer 2

Abid Rizvi 2018-2-16

3
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/315713-matlab-and-pendulum-function-need-help-thank-you#answer_305439

在 MATLAB Online 中打开

Try this code:

function [T] = pendulum(L,a0)
      T = 0;
      if L > 0
          dt = 1.e-6;
          a_velocity = 0; 
          g = 9.8;
          theta = abs(a0); 
          while theta > 0
              a_acceleration = g*sin(theta)/L;
              a_velocity = a_velocity + dt * a_acceleration;
              theta = theta - dt * a_velocity;
              T = T + dt;
          end
      T = T*4;
      end
  end

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Answer 3

Huan Yang 2017-6-8

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https://ww2.mathworks.cn/matlabcentral/answers/315713-matlab-and-pendulum-function-need-help-thank-you#answer_270047

function T=pendulum(L,a0) cta=a0; t=10^-6; w=0; T=0; while cta>0 T=T+t; a=-9.8/L*sin(cta); w=w+a*t; cta=cta+w*t; end

T=T*4;

end

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Hasan Emre Tekce 2017-8-29

在 MATLAB Online 中打开

Thanks Huan Yang but there is a lil bit mistake in this code. In order to fix this code to work properly is to just add to while loop L~= 0 since the division cannot be infinite and this is exact solution. 
 ....
while cta>0 && L~= 0   may fix your solution.

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Answer 4

Weronika Izdebska 2018-9-4

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/315713-matlab-and-pendulum-function-need-help-thank-you#answer_335346

function T = pendulum(L,angle0) T = 0; if L > 0 dt = 1e-6; g = 9.8; angle = abs(angle0); omega = 0; T = 0; while angle > 0 a = g*sin(angle)/L; omega = omega + dt * a; angle = angle - dt * omega; T = T + dt; end T = T * 4; end end

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