单摆运动模拟实现异常求助：代码逻辑问题排查

我正在学习实现单摆运动模拟，初始代码先施加重力，计算参考点到受重力影响后的端点向量，通过点积求解参考向量与变化向量的夹角，再以此角度设置摆锤位置，但运行结果异常。参考可汗学院相关文档修改代码后仍未解决问题，附上初始代码、修改后代码及异常效果动图，请求排查问题所在。

异常效果 #

初始代码 #

void SinglePendulum::setPendulum(double dt) {
    // Apply gravity
    auto acc = acceleration;
    Vec3<double> gravity(0.0, -9.8, 0.0);   // x = 0.0, y = -9.8, z = 0.0
    acc += gravity * inverseMass;           // get accelation
    velocity += acc * dt;                   // get velocity
    velocity *= pow(dampingCoeff, dt);      // apply damping
    variablePosition += velocity * dt;      // gravity-affected weight

    // Vector from reference point to end point after being affected by gravity
    variableVector = variablePosition - pivotPosition;        // a Position - b Position : b->a vector

    // The angle between the reference vector and the current vector
    double angle;
    angle = standardVector.Dot(variableVector);                             // dot product the reference vector and the variable vector
    double standVecLength = standardVector.Length();                        // magnitude of reference vector
    double variaVecLength = variableVector.Length();                        // magnitude of variable vector
    double finalAngle = acos(angle / (standVecLength * variaVecLength));    // get angle

    // Apply position with pendulum motion
    changePos.Set(sin(finalAngle) * length, cos(finalAngle) * length, 0.0); // apply x, y, z value
}

修改后代码 #

void SinglePendulum::setPendulum(double dt) {
    // The angle between the reference vector and the current vector
    double theta;
    theta = standardVector.Dot(variableVector);                             // dot product the reference vector and the variable vector
    double standVecLength = standardVector.Length();                        // magnitude of reference vector
    double variaVecLength = variableVector.Length();                        // magnitude of variable vector
    double angle = acos(theta / (standVecLength * variaVecLength));         // get angle

    Vec3<double> gravity(0.0, -9.8, 0.0);   // x = 0.0, y = -9.8, z = 0.0

    Vec3<double> forceX;
    Vec3<double> forceY;
    forceX.SetX(-9.8 * sin(angle));     // x = -9.8 * sin(angle), y = 0.0, z = 0.0
    forceY.SetY(-9.8 * cos(angle));     // x = 0.0, y = -9.8 * cos(angle), z = 0.0

    // Apply gravity
    auto acc = acceleration;
    acc += gravity * inverseMass;           // get accelation
    acc += forceX;
    acc += forceY;
    velocity += acc * dt;                   // get velocity
    velocity *= pow(dampingCoeff, dt);      // apply damping
    variablePosition += velocity * dt;      // gravity-affected weight

    // Vector from reference point to end point after being affected by gravity
    variableVector = variablePosition - pivotPosition;      // a Position - b Position : b->a vector
}

内容的提问来源于stack exchange，提问作者Keastmin