未定义对Main::playerX的引用

这是错误：

undefined reference to `Main::playerX'

有人能解释一下为什么会出错吗？

main.cpp（主函数）：

#include <GL/glut.h>
#include "Input.h"
#include "Main.h"
Input inputClass;
Main mainClass;
int main (int argc, char **argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode (GLUT_DOUBLE);
    glutInitWindowSize (800, 600);
    glutInitWindowPosition (100, 100);
    glutCreateWindow ("Your first OpenGL Window");
    glutDisplayFunc(mainClass.display);
    glutMainLoop();
}

输入.h:

#ifndef INPUT_H
#define INPUT_H
class Input
{
    public:
        Input();
        static void SpecialDown(int key, int x, int y);
        static void returnSpeed(float speed);
};
#endif // INPUT_H

Input.cpp:

#include "Input.h"
#include <GL/glut.h>
#include <GL/glu.h>
#include "Main.h"
Main mainClass2;
Input::Input()
{
    //ctor
}
void Input::SpecialDown(int key, int x, int y){
    switch(key){
        case GLUT_KEY_UP:
            returnSpeed(0.0f);
            break;
        case GLUT_KEY_DOWN:
            returnSpeed(0.0f);
            break;
        case GLUT_KEY_LEFT:
            returnSpeed(-0.005f);
            break;
        case GLUT_KEY_RIGHT:
            returnSpeed(0.005f);
            break;
    }
    //glutPostRedisp();
}
void Input::returnSpeed(float speed){
    mainClass2.test(speed);
}

Main.h:

#ifndef MAIN_H
#define MAIN_H

class Main{
    public:
        Main();
        static void reshape(int width, int height);
        static void display();
        void changeSpeed(float speed);
        static float playerX;
        void test(float kip);
        static float playerSpeed;
};
#endif // MAIN_H

Main.cpp:

#include "Main.h"
#include "Input.h"
#include <GL/glut.h>
#include <GL/glu.h>
Main::Main(){
    playerX = 0.0f;
}
Input input;
///HERE EVERYTHING IS BEING RENDERED
void Main::display (){
    ///BACKGROUND COLOR
    glClearColor(0.3f, 0.3f, 2.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    ///LOAD THE IDENTITY MATRIX FOR THE DRAWING
    glLoadIdentity();
    glTranslatef(0.0f, 0.0f, -5.0f);
    ///THE GROUND
    glBegin(GL_QUADS);
        glColor3f(0.5f,2.0f,0.5f);
        glVertex2f(-5.0f,-4.0f);
        glVertex2f(-5.0f,-1.0f);
        glVertex2f(10.0f,-1.0f);
        glVertex2f(10.0f,-4.0f);
    glEnd();
    glTranslatef(playerX,0.0f,0.0f);
    glBegin(GL_QUADS);
        glColor3f(1.0f,0.0f,0.0f);
        glVertex2f(-3.0f,-1.0f);
        glVertex2f(-3.0f,-0.5f);
        glVertex2f(-2.5f,-0.5f);
        glVertex2f(-2.5f,-1.0f);
    glEnd();
    ///SWAP THE BUFFERS FOR SMOOTH RENDERING
    glutSwapBuffers();
    playerX += playerSpeed;
}
void Main::test(float kip){
}

可能与静态变量playerX有关，但我不知道该怎么办。