I have, on occasion, installed an application to give it a try on my Mac, and when I attempt to drag it to the trash to delete it, I’m unable to do so: The Mac tells me that the application is still in use and I can’t delete it. I guess I could reboot my Mac (that would kill the application for sure), but is there an easier way?
Although installing random applications on the Mac just for playing around with is less dangerous than it is on Windows. It's still not something you should take lightly and I'd think hard about just installing some application to see what it does. But in any case if you want to uninstall an application by dragging it to the trash. You can't if the application is running.
You can't delete an application that's running and that's the problem. On the Mac you can run activity monitor and use that tool to shut down any application. To find activity manager. you can press command and space bar and then type activity manager or look in the utilities folder inside the Applications folder.
Once you run the application look for the application you're trying to delete in the list of running applications and you'll see there's a little red X button you can use to kill the application. Once you've done that you can drag it from the Applications folder to the Trash folder to get rid of it. If you find yourself wanting to delete applications often you might look into an application called App Delete. This is one of several available applications that deletes all the various bits and pieces of application that you install. Just dragging it to the trash doesn't necessarily remove all the extra little text files and descriptions of the application that end up clogging up your computer.
You can drag it to the trash that does uninstall it. App Delete will remove a few more files however. In any case use Activity Monitor to kill a running application so that you can delete it.