function phase_plot(F)

clf reset;

set(gcf,'nextplot','add',...
    'userdata',0);
set(gca,'nextplot','add',...
    'position',[0.1 .175 .8 .8]);

xlabel('x');ylabel('v'); 

Change_button=uicontrol('string','Go',...
    'units','normal',...
    'pos',[0.1 .02 .1 .06],...
    'callback','set(gcf,''userdata'',1)');

Beta_value=uicontrol('style','edit',...
    'units','normal',...
    'position',[.25 .02 .08 .06],...
    'string','b');

F_with_b=regexprep(F,'b','.1')
func=inline(F_with_b,'t','y','b');



while get(gcf,'userdata')==0
    pause(.25)

v0=4.5;

if get(gcf,'userdata')==1
    set(gcf,'Pointer','watch');
    new_b=get(Beta_value,'string');
    F_with_b=regexprep(F,'b',new_b);
    func=inline(F_with_b,'t','y','b');
    set(gcf,'userdata',0);
    cla;
    
    for i=1:6
    Initial_cond=[2*i-7;v0];
    [t,x]=ode45(func,[0,100],Initial_cond);
    plot(x(:,1),x(:,2));
    
    Initial_cond=[2*i-7;-v0];
    [t,x]=ode45(func,[0,100],Initial_cond);
    plot(x(:,1),x(:,2));
    
    Initial_cond=[.1;0];
    [t,x]=ode45(func,[0,100],Initial_cond);
    plot(x(:,1),x(:,2));
    
    x_limit=get(gca,'xlim');y_limit=get(gca,'ylim');
    x_bar=[x_limit(1):.1:x_limit(2)];zero_x=zeros(1,length(x_bar));
    y_bar=[y_limit(1):.1:y_limit(2)];zero_y=zeros(1,length(y_bar));
    hor_ver=plot(x_bar,zero_x,'black',...
        zero_y,y_bar,'black');
    set(hor_ver,'linewidth',.);
    end

end

new_b=get(Beta_value,'string');
set(gcf,'pointer','arrow')
title=['\beta=' new_b];
set(get(gca,'Title'),'string',title);

end