physics class returning weird numbers

I have been trying to make a physics class for to test on an gui, but when I get the output, it's not a # persay. I'm not even sure if the physics are right, and I will be looking into it, but I'm posting here to find out why i didn't get a #. If you see any problems with the physics part of it that would be greatly appreciated too.

Here is my code

Code:

#include <iostream>
#include <vector>

using namespace std;


vector <float> force;


class physics
{
    public:
    physics(double x, double y, double h, double w, double top_speed, double density, double friction_level);

    void apply_force(vector <float> force);
    private:
    vector <double> main_vector;
};
physics::physics(double x, double y, double h, double w, double top_speed, double density, double friction_level)
{
    //set up main vector
    main_vector.push_back(x);
    main_vector.push_back(y);
    main_vector.push_back(h);
    main_vector.push_back(w);
    main_vector.push_back(top_speed);
    main_vector.push_back(density);
    main_vector.push_back(friction_level);

    // find volume and mass
    // first we will find volume = h * w
    main_vector.push_back(main_vector[2] * main_vector[3]);

    // now lets find mass = V * D
    main_vector.push_back(main_vector[6] * main_vector[8]);

    // now we are going to find the gravity of the object using force = mass * 9.8(earth grav)
    main_vector.push_back(main_vector[9] * 9.8);

    // find friction
    main_vector.push_back(main_vector[6] * main_vector[10]);
    //lets make some room to put acceleration in there! remember we need one for x and y
    main_vector.push_back(0);
    main_vector.push_back(0);

}

void physics::apply_force(vector <float> force)
{
    // acceleration = force/ mass    * for both x and y
    // also not we subtract friction from it
    if(force[0]/main_vector[8] > 0)
    {
        main_vector[12] = (force[0] - main_vector[11] )/main_vector[8] ;
    }
    if(force[0]/main_vector[8] <= 0)
    {
        main_vector[12] = (force[0] - main_vector[11]) /main_vector[8] ;
    }

    if(force[1]/main_vector[8] > 0)
    {
        main_vector[13] = (force[1] - main_vector[11]) /main_vector[8];
    }

    if(force[1]/main_vector[8] <= 0)
    {
        main_vector[13] = (force[1] - main_vector[11]) /main_vector[8];
    }

    main_vector[0] += main_vector[12];
    main_vector[1] += main_vector[13];
    cout<< main_vector[0] << ", " << main_vector[1] << "\n";
}








int main()
{
    force.push_back(10);
    force.push_back(30);
    physics car(5, 5, 25, 25, 20, 40, 0.5);
    car.apply_force(force);

	return 0;
}

and my output is:
1.#INF, 1.#INF


Thanks for any help,

Joe

Are you dividing by zero?

[edit]Such that counting from zero you maybe ought to be doing this?

Code:

    // now lets find mass = V * D
    main_vector.push_back(main_vector[5] * main_vector[7]);  // 8

(For starters.)

This use of a vector seems dubious and confusing.