Mac OS X is almost always very stable, but sometimes you may encounter an app that is not so stable. Some apps may become frozen, unresponsive, or stuck. This short tip explains how to force an application to quit. Note that when you force close an application, any unsaved changes may not be saved. So force quit an app only if you are unable to quit in the usual way.
To force quit an application, navigate Apple menu > Force Quit (or you may press command-option-esc), then choose the app that you wish to close and click Force Quit (see IMG1 and IMG2).