double area, friction;

cout << "lutfen surtunen yuzeyi giriniz: n";
cin >> area;

cout << "lutfen surtunme katsayisini giriniz: n";
cin >> friction;

calc_force(velocity, area, friction);

// My function

void calc_force(int velocity, double area, double friction)
{
    double force;
    double seccade = friction;
    if(0.2 <= seccade && seccade <= 0.5 ) { // if  statement does not work
        force = (0.0615)*area*friction*velocity*velocity;
        cout << "Velocitytt Drag Coefficientt Areatt Forcen";
        cout << "tt" << velocity << "tt" << seccade << "tt" << area << "tt" << force << "n";
        velocity+=5;

        if(velocity>40) return ;

        cout << "lutfen surtunme katsayisini giriniz: n";
        cin >> friction;
        cout << "lutfen surtunen yuzeyi giriniz: n";
        cin >> area;
        calc_force(velocity, area, friction);
    } else { // this works
        cout << "lutfen surtunme katsayisini dogru giriniz: n";
        cin >> friction;
        cout << "lutfen surtunen yuzeyi giriniz: n";
        cin >> area;
        calc_force(velocity, area, friction);
    }
}

void calc_force(int velocity, double area, double friction){
    double force;
    double seccade = friction;
    if(0.2 <= seccade && seccade <= 0.5 ){ // if  statement does not work
        force = (0.0615)*area*friction*velocity*velocity;
        cout<<"Velocitytt Drag Coefficientt Areatt Forcen";
        cout<<"tt"<<velocity<<"tt"<<seccade<<"tt"<<area<<"tt"<<force<<"n";

        velocity+=5;
    }

    if (velocity > 40 ) {
        return;
    } else {
        cout<<"lutfen surtunme katsayisini giriniz: n";
        cin>>friction;
        cout<<"lutfen surtunen yuzeyi giriniz: n";
        cin>>area;
        calc_force(velocity, area, friction);
    }
}