Ode45 I get wrong results.

10 次查看（过去 30 天）
Arda Nova
Arda Nova 2018-12-31
评论： Arda Nova 2019-1-1
function dydt = idn (t, y)
dydt = zeros(2,1);
dydt(1)=y(2);
dydt(2)=-((9.81*sin(y(1)))/0.5);
dydt = [0.78 0];
[t1,y1] = ode45(@idn, [0 20], dydt);
plot(t1,y1(:,2));
grid on;
This is a code for simple pendulum. When I use this code group, acceleration comes out wrong(it goes between max 3.. when it should be 13.). What am I doing wrong? When I use sind and put 45 degrees instead of sin and 0.78 radia, this time I get 30 ish something as a max result when it shouldn't change.
Original Eqn: Theta''=-g*sin(Theta)/ l
  2 个评论
Arda Nova
Arda Nova 2018-12-31
编辑：Arda Nova 2018-12-31
dydt(2)=-((9.81*sin(y(1)))/0.5);
At the bottom, acceleration is maximum. If you put 270 degrees acceleration comes 19.62 rad/sec^2 (Sorry I had made a mistake at taking the degree. Tried it too, wrong result). One think is for sure, acceleration should not be affected from me using radian or degrees. I don't even know where does the program gets the 3.something. If I knew then I would maybe solve the issue but, couldn't figure it out.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Stephan
Stephan 2019-1-1
编辑：Stephan 2019-1-1
Hi,
ode45 integrates your second order ode two times. So the result of ode45 will be angle and velocity over time. To get acceleration you have to calculate it seperatly:
% initial conditions of angle and velocity
dydt = [pi/2; 0];
% integrat ode to get angle an velocity
[t1,y1] = ode45(@idn, [0 20], dydt);
% calculate acceleration
y1(:,3) = -9.81.*sin(y1(:,1))./0.5;
% plot results
plot(t1,y1(:,1),t1,y1(:,2),t1,y1(:,3));
grid on;
function dydt = idn(t, y)
dydt = zeros(2,1);
dydt(1)=y(2);
dydt(2)=-((9.81*sin(y(1)))/0.5);
end
Best regards
Stephan
  1 个评论
Arda Nova
Arda Nova 2019-1-1
I am very thankful to you, good sir. You have no idea how much you have helped.

请先登录，再进行评论。

更多回答（0 个）

类别

在 Help Center 和 File Exchange 中查找有关 Ordinary Differential Equations 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by