The best time to teach something new is usually when the period has just started and students are freshest.  A Launch is a question or a pattern or something to think on to introduce a new concept.

In contrast,  a Do Now is usually ties to something they already know.  A Do Now can be great for starting the class with a routine so students settle into class. although sometimes the Do Now can take quite a while and “Now” can become “Later”.  If you must have a Do Now, keep it short and sweet and try the Launch right afterwards.

Do you want your students to know what you are going to do before you do it?  In some schools, the culture is to have routines in the classroom and I can understand that perspective.  But too often the Do Now is followed by going over Homework and the opportunity to learn new material is put off until there is not much time left in the period.

Here is an example of a Launch for Exponential Functions   from the most recent College Algebra lesson.

Enjoy these definitions of launch and feel f any launc( http://www.thefreedictionary.com/launch)

launch ¹

(lônch, länch)