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02 December 2013

The first day of motion

On the first day of studying motion in conceptual physics, I start the class by having each student construct position-time graphs from motion diagrams.  The motion diagrams are made using spark timers and constant-speed carts, as shown in the picture.  

Spark timers are easy enough to use that I can just let them go at it with minimal instruction or supervision -- thread the paper through the machine, tape it to the cart, and voila.  Since the spark timer makes 10 dots per second, it's pretty straightforward to construct the position-time graph for the first 2 s of motion, because each dot simply represents 0.1 s.

(As an aside, if you have the older "ticker-tape machine" device that makes 60 impressions per second using honest-to-goodness carbon paper, you can just have students plot every sixth dot as 0.1 s.  It only took me eighteen years of teaching before I figured out that I didn't have to plot the time axis every 0.017 s.  And I didn't really figure it out, my colleague Curtis mentioned it to me, 'cuz I certainly never thought of that.  Guh.)

Once a group has a position-time graph constructed, I hand out just the first eight facts from my motion fact sheet and the problem set based on the first position-time graph.  (My original has a scaled grid for them to re-graph their data individually.  The scaled grid didn't come across on google docs... so feel free to email me for an original .doc copy.)  Everyone answers the questions with specific reference to the facts on the sheet.

Note that I do absolutely no lecture.  I've found that trying to tell students how to interpret motion diagrams and graphs is as useful as telling someone who's never played or watched football how a zone blitz works.  Nevertheless, it's amazing how quickly everyone figures out the meaning of various representations of motion when they are personally involved in creating those representations.  I eventually demand a nearly college-level understanding of motion graphs; but I get there by building from the ground up over many class periods.

The next day, or whenever a group is finished, we do the same exercise again, except this time with a PASCO cart sliding down an inclined track so that the cart is speeding up.  They answer similar questions about their graph.  Then we're off and running to use position-time graphs fluently to represent motion.


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