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## 09 January 2013

### Posted: The units of acceleration are...

On our school email server, I've created a conference for my classes.  Students are expected to read messages posted to this folder on a regular basis.  They're also encouraged to post questions for me or for each other, though they don't do that so much.  I post messages to the class, including electronic copies of the problem sets, announcements of upcoming assignments, and my thoughts as I grade student work.

The trick, of course, is to get the students to actually read messages, and to read them all the way through.

Occasionally I've put a statement at the bottom of a long message saying something like "Extra credit will be awarded to each student who hands me a folded up piece of paper in class with the word 'cow' on it. You may not discuss this extra credit opportunity with anyone."  That's been effective... I certainly have gotten better attention since I started occasionally including these "easter eggs."

Tonight I graded a test correction in which way too many students said "Objects in free-fall gain or lose 10 m/s of speed every second.  At the top of its flight, the ball is still in free fall; so its* acceleration must still be 10 m/s."

* Sanitized -- a ninth grade would, of course, write "it's."

Aarrgh.  Instead of pulling my hair out about the units of acceleration, I just posted a different sort of easter egg:

Consider your test correction to problem 7.  The problem states, when a ball thrown upward at 30 m/s reaches the peak of its flight, what is the ball's acceleration?

Many of you said that the acceleration is 10 m/s.  That's not right -- the units of acceleration are m/s per second.  The acceleration of an object in free fall, even at the top of its flight, is 10 m/s per second.

I will give you an extra point on tomorrow's quiz if you write at the bottom "The units of acceleration are m/s per second.  The units of speed are m/s."

This is even better than asking students to write some random code word to prove that they read my email; they have to write the correct units for acceleration, which can not be done enough times.