Question

当我运行此代码时，双摆显示一瞬间然后消失。似乎公式中的大多数变量num1，num2等在钟摆消失后的瞬间开始返回nan。谁能明白为什么？我认为这与将度数转换为180度的弧度有关，所以我在转换函数中添加了if语句。

这是Double Pendulum运动公式的链接。为简单起见，我将公式分解成碎片。 https://www.myphysicslab.com/pendulum/double-pendulum-en.html

这个想法来自Coding Train的Double Pendulum视频：https://www.youtube.com/watch?v=uWzPe_S-RVE

#undef __STRICT_ANSI__
#include "window.h"
#include <SFML/Graphics.hpp>
#include <GL/glew.h>
#include <cmath>
#include <iostream>

namespace Window
{
    sf::ContextSettings settings;

    sf::RenderWindow window(sf::VideoMode(600, 600), "Window", sf::Style::Close | sf::Style::Titlebar, settings);

    void init()
    {
        settings.depthBits = 24;
        settings.majorVersion = 4;
        settings.minorVersion = 6;  //OpenGL 4.6

        glewInit();
        glViewport(0,0, 600, 600);
    }

    void close()
    {
        window.close();
    }

    void update()
    {
        window.display();
    }

    void checkForClose()
    {
        sf::Event windowEvent;
        while (window.pollEvent(windowEvent))
        {
            if (windowEvent.type == sf::Event::Closed)
            {
                close();
            }
        }
    }

    bool isOpen()
    {
        return window.isOpen();
    }

    double convDeg(double deg)
    {
        if (deg == 180.0)
        {
            return 0.0;
        }

        else
        {
            return deg * M_PI / 180.0;
        }
    }

    void runLoop()
    {
        double r1 = 100;
        double m1 = 40;
        double a1 = 90;
        double a1_v = 0;

        double r2 = 100;
        double m2 = 40;
        double a2 = 30;
        double a2_v = 0;

        double x1 = 0;
        double x2 = 0;
        double y1 = 0;
        double y2 = 0;

        double g = 0.005;

        double num1 = 0;
        double num2 = 0;
        double num3 = 0;
        double num4 = 0;
        double den = 0;

        double a1_a = 0;
        double a2_a = 0;

        while (window.isOpen())
        {
            num1 = -g * (2 * m1 + m2) * std::sin(convDeg(a1));
            num2 = -m2 * g * std::sin(convDeg(a1-2*a2));
            num3 = -2*std::sin(convDeg(a1-a2))*m2;
            num4 = a2_v*a2_v*r2+a1_v*a1_v*r1*std::cos(convDeg(a1-a2));
            den = r1 * (2*m1+m2-m2*cos(convDeg(2*a1-2*a2)));
            a1_a = (num1 + num2 + num3*num4) / den;

            num1 = 2 * std::sin(convDeg(a1-a2));
            num2 = (a1_v*a1_v*r1*(m1+m2));
            num3 = g * (m1 + m2) * std::cos(convDeg(a1));
            num4 = a2_v*a2_v*r2*m2*std::cos(convDeg(a1-a2));
            den = r2 * (2*m1+m2-m2*std::cos(convDeg(2*a1-2*a2)));
            a2_a = (num1*(num2+num3+num4)) / den;

            x1 = 300 + (r1 * std::sin((convDeg(a1))));
            y1 = 300 + (r1 * std::cos((convDeg(a1))));
            x2 = x1 + (r2 * std::sin((convDeg(a2))));
            y2 = y1 + (r2 * std::cos((convDeg(a2))));

            a1_v += a1_a;
            a2_v += a2_a;
            a1 += a1_v;
            a2 += a2_v;

            sf::Vertex pOne[] =
            {
                sf::Vertex(sf::Vector2f(300,300)),
                sf::Vertex(sf::Vector2f(x1,y1)),
            };

            sf::Vertex pTwo[] =
            {
                sf::Vertex(sf::Vector2f(x1,y1)),
                sf::Vertex(sf::Vector2f(x2,y2)),
            };

            window.clear();
            window.draw(pOne, 2, sf::Lines);
            window.draw(pTwo, 2, sf::Lines);

            update();
            checkForClose();

            std::cout << std::sin(convDeg(a1)) << "\n";

        }
    }
}

Answer 1

问题清单：

convDeg：180度和0度不相等。
角度变量：加速度/力度a1_a, a1_v等在弧度中，而角度变量a1, a2等在度中。每次循环运行时，您的代码都会错误地将度数转换为弧度。在主循环之前进行转换。

（基于之前答案的推测 - 我可能错了）MyPhysicsLab上的公式似乎与其他来源的公式相矛盾，例如： here和here;使用拉格朗日力学的快速计算同意这两个来源并且不 MPL。

Double Pendulum返回nan结果。为什么？

1 个答案: