void ApplyExternalForce(World& world,
                        float xforce,
                        float yforce)
{
    vector<Particle>& particles = world.GetParticles();
    
    for(int i = 0; i < particles.size(); i++)
    {
        Particle& p = particles[i];
        
        if(xforce > 0)
        {
            if(p.r.x < -0.07)
                p.a.x += xforce;
        }
        else
        {
            if(p.r.x > 0.07)
                p.a.x += xforce;
        }
        
        if(yforce > 0)
        {
            if(p.r.z > 0.25)
                p.a.z += -yforce;
        }
        else
        {
            if(p.r.z < -0.25)
                p.a.z += -yforce;
        }
    }
}